Objectives
• Describe the connection
between substances and
elements. (17.1)
1
THE BIG
IDEA
• Give examples that illustrate
the small size of atoms. (17.2)
• Compare the ages of atoms to
the ages of the materials they
compose. (17.3)
• State evidence for the existence
of atoms. (17.4)
• Describe molecules. (17.5)
• Describe how compounds are
different from their component
elements. (17.6)
• Describe the distribution of
mass in an atom. (17.7)
• Explain the cause of an atom’s
chemical properties. (17.8)
• Identify the four phases of
matter. (17.9)
THE ATOMIC
NATURE OF MATTER
........
THE
ATOMIC NATURE
OF MATTER
Atoms are the building blocks
of most matter.
S
uppose you break apart a large boulder with a heavy sledgehammer. You
break the boulder into rocks. Then you
break the rocks into stones, and the stones
into gravel. You keep going and break the
gravel into sand, and the sand into a powder
of fine crystals. Each fine crystal is composed
of many billions of smaller particles called
atoms. Atoms are the building blocks of most
matter. Everything you see, hear, taste, feel,
or smell in the world around you is made
of atoms. Shoes, ships, mice, lead, and people
are all made of atoms.17.0
discover!
water, milk, needle,
slide, cover slip, microscope
MATERIALS
EXPECTED OUTCOME Fat
globules will move randomly
around.
ANALYZE AND CONCLUDE
1. The droplets moved around
randomly.
discover!
2. The chalk dust would also
move around randomly.
How Do We Know That Atoms Exist?
Analyze and Conclude
1. Place a drop of water on a microscope slide.
2. Add a drop of whole milk to the water drop
using a wire or needle and stir.
3. Place a cover slip on the slide.
4. Insert the slide into a microscope and wait a
while for the milk to settle.
5. Focus on fat globules in the milk. Start with a
low magnification and then switch to a higher
one.
1. Observing What did you see when
you viewed the fat droplets under
the microscope?
2. Predicting What would happen if you
replaced the fat globules with chalk dust?
3. Making Generalizations What does the
motion of the milk fat droplets tell you about
the surrounding water?
3. There are small particles
in the water undergoing
random motion, hitting the
milk droplets.
The random
motion of small particles
in water is the direct
consequence of the motion
of neighboring atoms and
molecules.
TEACHING TIP
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324
17.1 Elements
This chapter is the most
important chapter in Unit II,
and should not be skipped. It
provides a good background
for the chapters on heat
(Chapters 21–24) and also
provides the necessary
background for Chapters 28,
32, and all of Unit VI.
Just as dots of light of only three colors combine to form almost
every conceivable color on a television screen, only about 100 distinct
kinds of atoms combine to form all the materials we know about.
Atoms are the building blocks of matter. A material composed of
only one kind of atom is called an element.
To date about 115 elements are known. Of these, about 90 occur
in nature. The others are made in the laboratory with high-energy
atomic accelerators and nuclear reactors. These laboratory-produced
elements are too unstable (radioactive) to occur naturally in appreciable amounts.
Every simple, complex, living, or nonliving substance in the
known universe is put together from a pantry containing less than
100 elements. More than 99% of the material on Earth is formed
from only about a dozen of the elements. The other elements are
relatively rare. Living things, for example, are composed primarily of
five elements: oxygen (O), carbon (C), hydrogen (H), nitrogen (N),
and calcium (Ca). The letters in parentheses represent the chemical
symbols for these elements. Table 17.1 lists the 16 most common elements on Earth. Most of these elements, not just the five most common ones, are critical for life.
The lightest element of all is hydrogen. In the universe at large, it
is the most abundant element—over 90% of the atoms in the known
universe are hydrogen. Helium, the second-lightest element, makes
up most of the remaining atoms in the universe, although it is rare
on Earth. The heavier, naturally formed atoms that we find around
us were manufactured by fusion reactions in the hot, high-pressure
cauldrons deep within stars. Elements heavier than iron are formed
when huge stars implode and then explode—an event called a supernova. The heaviest elements are formed when pairs of neutron stars,
the super-dense cores of supernovas, collide. Nearly all the atoms
on Earth are remnants of stars that exploded long before the solar
system came into being.
CHAPTER 17
17.1 Elements
Key Terms
atoms, element
Common Misconception
Material things are made of
thousands of different kinds of
atoms.
FACT To date, there are only
about 120 known elements.
Just as we don’t own
the atoms in our bodies, we don’t own
energy—we rent it.
Much of the energy we
receive from the sun
is eventually radiated
back into space.
THE ATOMIC NATURE OF MATTER
Teaching Tip Begin by
describing breaking a boulder
into rocks, rocks into stones,
stones into pebbles, pebbles into
gravel, gravel into sand, sand
into powder, and so forth until
you get to the fundamental
building block—the atom. Give
examples to convey the idea of
the smallness of the atom, i.e.,
an atom is as many orders of
magnitude smaller than a person
as an average star is larger than
a person. The size of an atom is
to the size of an apple as the size
of an apple is to the size of Earth.
An apple is full of as many atoms
as Earth would have apples if it
were packed solid with apples.
325
325
The lovely girl in Figure 17.1
is my daughter Leslie when she
was 16. She is now a geologist
and a teacher.
Teaching Tip Stress that the
fusing of hydrogen nuclei to
form nuclei of other elements is
very different from the formation
of compounds as discussed in
Section 17.6. In fusion, the nuclei
actually merge and become one,
whereas in compound formation,
the individual nuclei do not
change.
FIGURE 17.1 Both Leslie and you are
made of stardust—in the
sense that the carbon,
oxygen, nitrogen, and
other atoms that make up
your body originated in the
deep interior of ancient
stars, which have long since
exploded.
......
Every simple,
complex, living, or
nonliving substance in the known
universe is put together from a
pantry containing less than
100 elements.
CONCEPT
CHECK
All of the matter that we encounter in our daily lives, as well as
matter in the sun and other stars, is made up of elements. But not
all matter in the universe is composed of elements. In the closing
years of the twentieth century, astrophysicists found that gravitational forces within galaxies were far greater than visible matter could
account for. Only in this twenty-first century has it been confirmed
that some twenty-three percent of the matter in the universe is composed of an unseen dark matter. Astrophysicists believe this dark
matter is made up of particles not yet detected, and that much of the
rest of the universe is dark energy (briefly mentioned in Chapter 9).
Indeed, the nature of dark matter, which gravitationally affects ordinary matter, and dark energy, which pushes outward on the expanding universe, is the focus of enormous present-day research.
discover!
small container,
water, food coloring
MATERIALS
EXPECTED OUTCOME Students
should observe that the rate
of migration is higher in the
hotter water.
The molecules in the
hot water move faster.
......
THINK
CONCEPT
CHECK
Teaching Resources
If a typical atom were
expanded to a diameter
of 3 km, about the size
of a medium-sized airport, the nucleus would
be about the size of a
basketball. Atoms are
mostly empty space.
• Reading and Study
Workbook
• PresentationEXPRESS
• Interactive Textbook
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326
What do all substances have in common?
discover!
How fast do atoms migrate?
1. Put some drops of food coloring into a small container of water.
How long does it take for the colored drops to spread to all parts
of the water?
2. Repeat, using hot water. What happened to the migration rate of
the drops?
3. Think Why did the rate change?
17.2 Atoms Are
Small
17.2 Atoms Are Small
23
......
CONCEPT
CHECK
How small are atoms?
CHAPTER 17
FIGURE 17.2 There are as many atoms
in a normal breath of air as
there are breathfuls of air
in the atmosphere of the
world.
think!
Does your brain contain
atoms that were once
part of Albert Einstein?
Explain.
Answer: 17.2
Teaching Tip State that if you
put a drop of ink in a bathtub
full of water, you can very soon
sample any part of the water
and find ink in it. The atoms of
ink spread out. There are more
atoms in a thimbleful of ink than
there are thimblesful of water
in all the lakes and rivers of the
world. That means if you throw
a thimbleful of ink into one of
the Great Lakes, eventually it
will mix, and then if you dip
anywhere in the lake with a
thimble, you’ll have many atoms
of ink in your sample.
Teaching Tip Point out
that atoms are so tiny that you
inhale billions of trillions with
each breath. Assuming that
most of the atoms previously
breathed by people are still part
of the atmosphere, then you
inhale billions of atoms exhaled
by nearly every person who ever
breathed! So in this sense we are
all one!
FIGURE 17.3 Information about the ship
is revealed by passing
waves, because the distance between wave crests
is small compared with the
size of the ship. The passing
waves reveal nothing about
the chain.
THE ATOMIC NATURE OF MATTER
327
Atoms are so small
that there are about
1023 atoms in a gram of water (a
thimbleful).
......
Atoms are so small that there are about 10 atoms in a gram
of water (a thimbleful). The number 1023 is an enormous number,
more than the number of drops of water in all the lakes and rivers
of the world. So there are more atoms in a thimbleful of water than
there are drops of water in the world’s lakes and rivers. Atoms are so
small that there are about as many atoms in the air in your lungs at
any moment as there are breathfuls of air in the atmosphere of the
whole world.
Atoms are perpetually moving. They also migrate from one location to another. In solids the rate of migration is low, in liquids it is
greater, and in gases migration is greatest. Drops of food coloring in a
glass of water soon spread to color the entire glass of water. Likewise,
a cupful of toxic material thrown into an ocean spreads around and
is eventually found in every part of the world’s oceans. The same is
true of materials released into the atmosphere.
It takes about six years for one of your exhaled breaths, such as
the one in Figure 17.2, to become evenly mixed in the atmosphere.
At that point, every person in the world inhales an average of one
of your exhaled atoms in a single breath. And this occurs for each
breath you exhale! When you take into account the many thousands
of breaths that people exhale, at any time—like right now—you
have hordes of atoms in your lungs that were once in the lungs of
every person who ever lived. We are literally breathing one another’s
breaths.
Atoms are too small to be seen—at least with visible light. You
could connect an array of optical microscopes atop one another and
never “see” an atom. This is because light is made up of waves, and
atoms are smaller than the wavelengths of visible light. The size of
a particle visible under the highest magnification must be larger than
the wavelengths of visible light. This is better understood by an
analogy with water waves. A ship is much larger than the water waves
that roll on by it. As Figure 17.3 shows, water waves can reveal features of the ship. They diffract as they pass the ship. In contrast, diffraction is nil for waves that pass the anchor chain, revealing little or
nothing about it. Similarly, waves of visible light are too coarse compared with the size of an atom to show details of the atom’s size and
shape. Atoms are incredibly small. (More about this in Chapter 31.)
Teaching Tip On the board,
write a 1 followed by 23 zeros to
show the meaning of 1023. State
that the number of molecules in
a liter of air is about 1023. The
number of liters of air in the
atmosphere is about 1023. Hence,
evenly mixed, after a single
breathful of air spreads over the
atmosphere (about 6 years), on
average, one molecule of that
breath will be in each liter of air
in the atmosphere.
CONCEPT
CHECK
Teaching Resources
• Laboratory Manual 47
327
17.3 Atoms Are
Recyclable
Common Misconceptions
The atoms that make up a newborn
baby were made in the mother’s
womb.
FACT The atoms that make up a
developing baby in the mother’s
womb are supplied by the food
the mother eats. These atoms
were formed in stars that have
long since exploded.
think!
World population grows
each year. Does this
mean the mass of Earth
increases each year?
Explain.
Answer: 17.3
The age of atoms in a baby is less
than the age of atoms in an old
person.
FACT The atoms that make up a
baby are the same age as those
that make up everybody, no
matter what their age.
......
Atoms in your body
have been around
since long before the solar system
came into existence, more than
4.6 billion years ago.
CONCEPT
CHECK
17.3 Atoms Are Recyclable
Atoms are ageless and are much older than the materials they compose. Some atoms are nearly as old as the universe itself. Most atoms
that make up our world are at least as old as the sun and Earth.
Atoms in your body have been around since long before the
solar system came into existence, more than 4.6 billion years ago.
They cycle and recycle among innumerable forms, both living and
nonliving. Every time you breathe, for example, only some of the
atoms that you inhale are exhaled in your next breath. The remaining atoms are taken into your body to become part of you, and most
leave your body sooner or later—to become part of everything else.
Strictly speaking, you don’t “own” the atoms that make up your
body—you’re simply their present caretaker. There will be many others who later will care for the atoms that presently compose you. We
all share from the same atom pool, as atoms migrate around, within,
and throughout us. So some of the atoms in the ear you scratch today
may have been part of your neighbor’s breath yesterday!
Most people know we are all made of the same kinds of atoms.
But what most people don’t know is that we are made of the same
atoms—atoms that cycle from person to person and creature to creature as we breathe and perspire.
......
CONCEPT For how long have the atoms in your body
CHECK
Teaching Resources
• Reading and Study
Workbook
17.4 Evidence for Atoms
• PresentationEXPRESS
• Interactive Textbook
• Conceptual Physics Alive!
DVDs Atoms
17.4 Evidence for
Atoms
Key Term
Brownian motion
The jittery motion of
a huge balloon in the
midst of a soccer field
filled with jostling
people would look like
Brownian motion from
a high-flying aircraft.
The people may be too
small to see, but not
the larger balloon.
Demonstration
Demonstrate Brownian motion
by putting a tiny amount of
lycopodium in some water and
have students observe a drop
under a microscope.
328
been around?
328
The idea that matter is made of atoms goes back to the Greeks in the
400s B.C. It was revived in the early 1800s by an English meteorologist and school teacher, John Dalton. He explained the nature
of chemical reactions by proposing that all matter is made of atoms,
but he had no direct evidence for their existence. The first fairly
direct evidence for the existence of atoms was unknowingly discovered in 1827. A Scottish botanist, Robert Brown, was looking
through a microscope to study pollen grains floating in water. He
noticed that the grains were in a constant state of agitation, always
jiggling about. At first, Brown thought that the grains were some
sort of moving life forms. Later, he found that inanimate dust particles and grains of soot floating in water also showed this kind of
motion. Brownian motion is the perpetual jiggling of particles that
are just large enough to be seen.
Brownian motion is evidence that atoms exist, as it results
from the motion of neighboring atoms and molecules. They bump
into the larger particles we can see. More direct evidence for the
existence of atoms is available today. An image of individual atoms
is shown in Figure 17.4. The image was made not with visible light
but with an electron beam. A familiar example of an electron beam is
the one that sprays the picture on some television screens. Although
an electron beam is a stream of tiny particles (electrons), it has wave
properties, with a wavelength more than a thousand times smaller
than the wavelength of visible light. With such a beam, atomic detail
can be seen. The historic (1970) image in Figure 17.4 was taken with
a very thin electron beam in a scanning electron microscope. It is the
first such image of clearly distinguishable atoms.
In the mid-1980s, researchers developed a different kind of microscope—the scanning tunneling microscope, small enough to be held
in your hand. In Figure 17.5, you can see individual atoms. Even
greater detail is possible with newer types of imaging devices that are
presently revolutionizing microscopy.
We can’t see inside atoms, but images with today’s devices help
us to construct better models of the atom. From these models we can
make predictions about unseen portions of the natural world.
Teaching Tip Tell students
that the first real “proof” for
atoms was given by Einstein in
1905, the same year he published
his paper on relativity. He
calculated what kind of motion
there ought to be in Brownian
motion, based on ideas such
as energy and momentum
conservation, and the idea of
heat as atomic motion.
FIGURE 17.4 The strings of dots are
chains of thorium atoms
imaged with a scanning
electron microscope by
researchers at the University
of Chicago’s Enrico Fermi
Institute.
Teaching Tip Ask what
an atom would “look like” if
viewed through a vertical bank
of about 40 high-powered optical
microscopes stacked one atop the
other. Then explain that atoms
don’t look like anything—they
have no appearance in the range
of frequencies we call light.
Teaching Tip Discuss Figures
17.4 and 17.5. These techniques
have opened new doors in fields
such as medicine and biology.
The positions of atoms in
complex molecules are no longer
guesswork.
CHECK
existence of atoms?
Atoms are in a state
of perpetual motion—
moving all the time.
Brownian motion is
CHECK evidence that atoms
exist, as it results from the
motion of neighboring atoms
and molecules. They bump into
the larger particles we can see.
......
......
CONCEPT How does Brownian motion provide evidence for the
CONCEPT
Teaching Resources
FIGURE 17.5
A scanning tunneling
microscope created this
image of uranium atoms.
• Reading and Study
Workbook
• PresentationEXPRESS
• Interactive Textbook
CHAPTER 17
THE ATOMIC NATURE OF MATTER
329
329
17.5 Molecules
17.5 Molecules
Key Term
molecule
Atoms can combine to form larger particles called molecules.
A molecule is the smallest particle of a substance consisting of two
or more atoms that bond together by sharing electrons. Molecules
can be made up of atoms of the same element or of different
elements. For example, two atoms of hydrogen (H) combine with a
single atom of oxygen (O) to form a water molecule (H2O). The gases
nitrogen and oxygen, which make up most of the atmosphere, are
both made of simple two-atom molecules (N2 and O2). In contrast,
the double helix of deoxyribonucleic acid (DNA), the blueprint of
life, is composed of millions of atoms.
Teaching Tip Distinguish
between atoms and molecules.
There is a limited number of
different atoms, but there
are innumerable different
molecules—and more are being
discovered and constructed.
Ask How many elements
are in a water molecule? Two:
hydrogen and oxygen How many
atoms are in a water molecule?
Three: two of hydrogen and one
of oxygen
Teaching Tip Point out
that whereas an individual
atom cannot be seen by the
naked eye, some molecules can.
One such molecule, called a
macro-molecule, is a diamond.
A diamond is actually one big
carbon molecule!
FIGURE 17.6 Models of the simple molecules O2 (oxygen gas), NH3
(ammonia), and CH4 (methane) show their structure.
The atoms that compose a
molecule are not just mixed
together, but are bonded in
a well-defined way.
Demonstration
Open a bottle of strong
perfume in one corner of the
room. Ask students to raise
their hands as soon as they
smell it. This will give your
class a good indication of the
speed of dispersion of the
perfume molecules.
......
Molecules can be
made up of atoms of
the same element or of different
elements.
CONCEPT
CHECK
FIGURE 17.7 Teaching Resources
• Reading and Study
Workbook
• Laboratory Manual 48
• PresentationEXPRESS
A scientist used an electron
microscope to take this
photograph of rubella virus
molecules. The white dots
are the virus erupting on the
surface of an infected cell.
......
• Interactive Textbook
Matter that is a gas or liquid at room temperature is usually made
of molecules. Matter made of molecules may contain all the same
kind of molecule, or it may be a mixture of different kinds of molecules, as shown in Figure 17.6. Purified water contains almost
entirely H2O molecules, whereas clean air contains molecules belonging to several different substances.
But not all matter is made of molecules. Metals and crystalline
minerals (including common table salt) are made of atoms that are
not joined in molecules.
Like atoms, individual molecules are too small to be seen with
optical microscopes.17.5 More direct evidence of tiny molecules is seen
in electron microscope photographs. The photograph in Figure 17.7
is of virus molecules, each composed of thousands of atoms. These
giant molecules are visible with a short-wavelength electron beam,
but are still too small to be seen with visible light.
We are able to detect some molecules through our sense of smell.
Noxious gases such as sulfur dioxide, ammonia, and ether are clearly
sensed by the organs in our nose. The smell of perfume is the result
of molecules that rapidly evaporate from the liquid and jostle around
completely haphazardly in the air until some of them accidentally
get close enough to our noses to be inhaled. The perfume molecules
are certainly not attracted to our noses! They wander aimlessly in all
directions from the liquid perfume to become a small fraction of the
randomly jostling molecules in the air.
CONCEPT
CHECK
330
330
What are molecules made of?
17.6 Compounds
17.6 Compounds
Key Terms
compound, chemical formula
......
Compounds have
properties different
from those of the elements of
which they are made.
......
A compound is a substance that is made of atoms of different elements combined in a fixed proportion. The chemical formula of the
compound tells the proportions of each kind of atom. For example, in
the gas carbon dioxide, the formula CO2 indicates that for every carbon (C) atom there are two oxygen (O) atoms. Water, table salt, and
carbon dioxide are all compounds. Air, wood, and salty water are not
compounds, because the proportions of their atoms vary.
A compound may or may not be made of molecules. Water and
carbon dioxide are made of molecules. On the other hand, table salt
(NaCl) is made of different kinds of atoms arranged in a regular pattern. (We’ll soon see that the atoms in NaCl are actually ions.) Every
chlorine atom is surrounded by six sodium atoms, as shown in Figure
17.8. In turn, every sodium atom is surrounded by six chlorine atoms.
As a whole, there is one sodium atom for each chlorine atom, but
there are no separate groups that can be labeled molecules.
Compounds have properties different from those of the elements of which they are made. At ordinary temperatures, water is a
liquid, whereas hydrogen and oxygen are both gases. Table salt is an
edible solid, whereas chlorine is a poisonous gas.
CONCEPT
CHECK
Teaching Resources
FIGURE 17.8 Table salt (NaCl) is a compound that is not made of
molecules. The sodium and
chlorine ions are arranged
in a repeating pattern.
Each ion is surrounded by
six ions of the other kind.
• Interactive Textbook
17.7 The Atomic
Key Terms
nucleus, nucleons, neutrons,
protons, isotopes, atomic number
Teaching Tip Stress the
emptiness of the atom and lead
into the idea of solid matter
being mostly empty space. State
that our bodies are 99.999%
empty spaces, and that a particle,
if tiny and not affected by
electrical forces, could be shot
straight through us without
even making a hole! Neutrons
do just that. From a beam of
neutrons, a few may make
bull’s-eye collisions with some of
our atomic nuclei—this can do
damage, so we wouldn’t want to
really do this. However, all but
a minute fraction in a beam of
neutrons would pass unhindered
through the body.
component elements?
17.7 The Atomic Nucleus
An atom is mostly empty space. Almost all of an atom’s mass is
packed into the dense central region called the nucleus. The New
Zealander physicist Ernest Rutherford discovered this in 1911 in his
now-famous gold foil experiment. Rutherford’s group shot a beam of
charged particles (alpha particles) from a radioactive source through
a thin gold foil. They measured the angles at which the particles were
deflected from their straight-line paths when they emerged. This was
accomplished by noting spots of light on a zinc-sulfide screen that
nearly surrounded the gold foil as shown in Figure 17.9.
FIGURE 17.9
The occasional large-angle
scattering of alpha particles
from the gold atoms led
Rutherford to the discovery of
the small, very massive nuclei
at their centers.
CHAPTER 17
• PresentationEXPRESS
Nucleus
CONCEPT How are compounds different from their
CHECK
• Reading and Study
Workbook
THE ATOMIC NATURE OF MATTER
331
Teaching Tip Discuss
Rutherford’s discovery of the
nucleus (Figure 17.9), the Bohr
model of the atom (Figure
17.10), and the electrical role
of the nucleus and surrounding
electrons.
331
Teaching Tip Discuss the role
of electrical forces in preventing
us from oozing into our chairs
and so forth. Stand up on your
table. Ask your students to
imagine that it is a large magnet,
and that you wear magnetic
shoes that are repelled by the
table you “stand” on. Then
state that on the submicroscopic
scale that this is indeed what
happens when you walk on any
solid surface. Only the repelling
force isn’t magnetic, it’s electric!
Discuss the submicroscopic
notion of things touching.
Acknowledge that under very
special circumstances the nucleus
of one atom can actually touch
the nucleus of another atom—
that this is what happens in a
thermonuclear reaction.
Ask True or False? There
exists a large air gap between
the nucleus of an atom and the
orbiting electrons. False, there
is a void, but it is not an air gap.
Air is far from being a void,
and is a substance that consists
principally of nitrogen and
oxygen molecules—much too big
to fit between a nucleus and its
orbiting electrons.
Link to CHEMISTRY
Water What’s in a glass of water? Tap water is far from being pure H2O,
for it contains dissolved compounds of metals such as iron, potassium, and
magnesium; dissolved gases such as oxygen and nitrogen; trace amounts
of heavy metals and organic compounds; and other chemical compounds
such as calcium fluoride and chlorine disinfectants. Now don’t panic and go
thirsty. You probably wouldn’t like the taste of pure water, for some dissolved
substances give water a pleasing taste and promote good health. As much as
10% of our daily requirement of iron, potassium, calciuum, and magnesium is
obtained from ordinary drinking water. Bottoms up!
Atoms were a philosophical concept with
ancient Greeks and
became a scientific concept with the experiments of the chemist
John Dalton in the early
1800s. Atoms weren’t
fully validated until the
work of Albert Einstein
in the early 1900s.
For: Links on atoms
Visit: www.SciLinks.org
Web Code: csn – 1707
332
332
Most particles continued in a more or less straight-line path
through the thin foil. But, surprisingly, some particles were widely
deflected. Some were even scattered back almost along their incoming
path. It was as surprising, Rutherford said, as firing a 15-inch artillery
shell at a piece of tissue paper and having it come back and hit you.
Rutherford reasoned that within the atom there had to be a
positively charged object with two special properties. It had to be
very small compared with the size of the atom, and it had to be massive enough to resist being shoved aside by heavy alpha particles.
Rutherford had discovered the atomic nucleus.
The mass of an atom is primarily concentrated in the
nucleus. However, the nucleus occupies less than a trillionth of the
volume of an atom. Atomic nuclei (plural of nucleus) are extremely
compact and extremely dense. If bare atomic nuclei could be packed
against one another into a lump 1 cm in diameter (about the size of a
small grape), the lump would weigh about a billion tons!
Huge electrical forces of repulsion prevent such close packing of
atomic nuclei because each nucleus is electrically charged and repels
the other nuclei. Only under special circumstances are the nuclei of
two or more atoms squashed into contact. When this happens, the
violent reaction known as nuclear fusion takes place. Fusion occurs
in the core of stars and in a hydrogen bomb.
Nucleons The principal building blocks of the nucleus are
nucleons. 17.7 Nucleons in an electrically neutral state are neutrons.
Nucleons in an electrically charged state are protons. All neutrons
are identical; they are copies of one another. Similarly, all protons are
identical. Atoms of various elements differ from one another by their
numbers of protons. Atoms with the same number of protons all
belong to the same element.
Teaching Tip Tell students
that each of the common
elements carbon, oxygen, and
nitrogen has two or three
isotopes that are found in nature.
The carbon-14 isotope is used
in radioactive dating, as will be
described in Chapter 39.
Isotopes For a given element, however, the number of neutrons
will vary. Atoms of the same element having different numbers of
neutrons are called isotopes of that element. The nucleus of the
common hydrogen atom has a single proton. When this proton is
accompanied by a neutron, we have deuterium, an isotope of hydrogen. When two neutrons are in a hydrogen nucleus, we have the isotope tritium. Every element has a variety of isotopes. Lighter elements
usually have an equal number of protons and neutrons, and heavier
elements usually have more neutrons than protons.
Atomic Number Atoms are classified by their atomic number,
which is the number of protons in the nucleus. The nucleus of a
hydrogen atom has one proton, so its atomic number is 1. Helium
has two protons, so its atomic number is 2. Lithium has three protons, so its atomic number is 3, and so on, in sequence up to the
heaviest elements.
Electric Charge Electric charge comes in two kinds, positive and
negative. Protons in the atom’s nucleus are positive, and electrons
orbiting the nucleus are negative. Positive and negative refer to a
basic property of matter—electric charge. (Much more about electric
charge in Unit V.) Like kinds of charge repel one another and unlike
kinds attract one another. Protons repel protons but attract electrons.
Electrons repel electrons but attract protons. Inside the nucleus,
protons are held to one another by a strong nuclear force. This force is
extremely intense but acts only across tiny distances. (More about the
strong nuclear force in Chapter 39.)
Teaching Tip Van der Waals
force (the relatively weak
attractive force between nonpolar molecules) mainly accounts
for the adhesion of the many
ridges in the feet of a gecko.
How long would it
take to count to one
million (106)? If each
count takes one second,
counting nonstop to a
million would take 11.6
days. Counting to a billion (109) would take
31.7 years. Counting to
a trillion (1012) would
take 31,700 years, and
to a trillion trillion (1024)
would take about 2 million times the estimated
age of the universe.
......
CONCEPT Where is the mass of an atom primarily concen-
CHECK
trated?
Physics on the Job
......
The mass of an atom
CHECK is primarily
concentrated in the nucleus.
CONCEPT
Chemist Many of the products you use every day—from shampoo to
vitamins to some foods—were developed by chemists. A chemist
uses an understanding of atoms and elements to isolate and identify
unknown chemicals, synthesize chemicals in the laboratory, and
perform tests to maintain quality standards in industrial processes.
Chemists work for government and university laboratories as well as
for research departments of private corporations. Most chemists hold
undergraduate degrees in chemistry along with advanced degrees
in more specific fields such as biochemistry, nuclear chemistry, or
analytical chemistry.
Teaching Resources
• Reading and Study
Workbook
• Transparency 26
• PresentationEXPRESS
• Interactive Textbook
• Next-Time Questions 17-1,
17-2
CHAPTER 17
THE ATOMIC NATURE OF MATTER
333
333
17.8 Electrons in
17.8 Electrons in the Atom
the Atom
Key Terms
ion, shell model of the atom,
periodic table
Electrons that orbit the atomic nucleus are identical to the electrons
that flow in the wires of electric circuits. They are negatively charged
1
subatomic particles. The electron’s mass is less than 1800 the mass of a
proton or neutron, so electrons do not significantly contribute to the
atom’s overall mass.
Teaching Tip Schematically
show the hydrogen atom, and
add a proton and neutrons to
build a helium atom, then a
lithium atom, and so on. Discuss
atomic number, and the role
that the number of protons
plays in the nucleus in dictating
the surrounding electron
configuration.
Teaching Tip Call attention
to and briefly discuss the periodic
table on page 336. Point out
that the atomic configurations
depicted in Figure 17.11 are
simply models. Models are
not complete or accurate. For
example, if the nuclei were
drawn to scale they would be
scarcely visible specks. And the
electrons don’t actually orbit
like planets as the drawings
suggest—such terms don’t have
much meaning at the atomic
level.
FIGURE 17.10 The classic model of the
atom consists of a tiny
nucleus surrounded by
orbiting electrons.
In an electrically neutral atom, such as the one shown in Figure
17.10, the number of negatively charged electrons always equals the
number of positively charged protons in the nucleus. When the number of electrons in an atom differs from the number of protons, the
atom is no longer neutral and has a net charge. An atom with a net
charge is an ion.
Attraction between a proton and an electron can cause a bond
between atoms to form a molecule. For example, two atoms can be
held together by the sharing of electrons (a covalent bond). Atoms
also stick to each other when ions of opposite charge are formed, and
these ions are held together by simple electric forces (an ionic bond).
Just like our solar system, the atom is mostly empty space. The
nucleus and surrounding electrons occupy only a tiny fraction of
the atomic volume. Yet the electrons, because of their wave nature,
form a kind of cloud around the nucleus. Compressing this electron
cloud takes great energy and means that when two atoms come close
together, they repel each other. If it were not for this repulsive force
between atoms, solid matter would be much more dense than it is.
334
334
Electrons don’t exactly orbit
because we lose the distinction
between particle and wave.
Can we say they “swarm” or
“smear”? For your students
that continue studying physics,
they will later be up against the
concept that something can be
both a particle and a wave.
Teaching Tip Tell students
that if the nucleus of the atom
were a peanut on second base
in Yankee Stadium, the electron
cloud would extend 200 m in
every direction, encompassing
the whole stadium and reaching
as high as a 50-story building.
FIGURE 17.11 ......
The shell model of the atom
pictures the electrons orbiting in concentric, spherical
shells around the nucleus.
The periodic table is a
chemist’s road map.
The arrangement of
electrons in the shells
around the atomic nucleus dictates
the atom’s chemical properties.
......
We and the solid floor upon which we stand are mostly empty
space, because the atoms making up these and all materials are themselves mostly empty space. But we don’t fall through the floor. The
forces of repulsion keep atoms from caving in on one another under
pressure.
Scientists use a model to explain how atoms of different elements
interact to form compounds. In the shell model of the atom, electrons are pictured as orbiting in spherical shells around the nucleus,
as shown in Figure 17.11. There are seven different shells, and each
shell has its own capacity for electrons. The arrangement of electrons in the shells around the atomic nucleus dictates the atom’s
chemical properties. These properties include melting and freezing
temperatures, electrical conductivity, and the taste, texture, appearance, and color of substances. The arrangement of electrons quite
literally gives life and color to the world.
The periodic table is a chart that lists atoms by their atomic
number and by their electron arrangements as shown in Figure 17.12
on the next page. As you read across from left to right, each element
has one more proton and electron than the preceding element. As
you go down, each element has one more shell filled to its capacity
than the element above.
Elements in the same column have similar chemical properties,
reacting with other elements in similar ways to form new compounds
and materials. Elements in the same column are said to belong to the
same group or family of elements. Elements of the same group have
similar chemical properties because their outermost electrons are
arranged in a similar fashion.
CONCEPT
CHECK
CONCEPT What does the arrangement of electrons around the
CHECK
nucleus determine?
CHAPTER 17
THE ATOMIC NATURE OF MATTER
335
335
• Concept-Development
Practice Book 17-1
• Transparencies 27, 28
• PresentationEXPRESS
• Interactive Textbook
336
336
Note that the uppermost row consists of only two elements, hydrogen
and helium. The electrons of helium complete the innermost shell.
Elements are arranged vertically on the basis of similarity in the
arrangement of outer electrons, which dictates similarities in physical
and chemical properties of the elements and their compounds.
Teaching Tip State that the
configuration of electrons and
their interactions with each other
are basically what chemistry is
about.
Teaching Resources
• Reading and Study
Workbook
The periodic table of the elements. The atomic number, above the
chemical symbol, is equal to the number of protons in the nucleus
(and equivalently, the number of electrons that surround the nucleus
in a neutral atom). The number below is the atomic mass. Each row
in the periodic table corresponds to a different number of electron
shells in the atom.
FIGURE 17.12 Darmstadtium Roentgenium
Teaching Tip Emphasize
the “orderliness” of all the
elements. There is a pattern
and relationship between each
element and those surrounding it
in the periodic table.
17.9 The Phases of
Matter
17.9 The Phases of Matter
CONCEPT
CHECK
Key Term
plasma
Teaching Tip Briefly discuss
the phases of matter, and how
different molecular speeds
account for the solid, liquid,
gaseous, and plasma phases.
In Chapter 23, students will
learn how matter changes from
one phase to another.
Watch for superheated
plasma torches that
create more electricity
than they consume as
they incinerate trash,
making today’s landfills
history.
What are the four phases of matter?
FIGURE 17.13
The aurora borealis is
light given off by glowing
plasma. High-altitude gases
in the northern sky are
transformed into glowing
plasmas by the bombardment of charged particles
from the sun. Less spectacular plasmas are found in
glowing fluorescent tubes
and advertising signs.
Matter exists in four
phases: solid, liquid,
gaseous, and plasma.
......
......
Matter exists in four phases: solid, liquid, gaseous, and plasma.
In the plasma phase, matter consists of positive ions and free
electrons. These charged particles make plasma a great conductor
of electricity. The plasma phase exists only at high temperatures.
Although plasma is less common to our everyday experience, it is the
predominant phase of matter in the universe. The sun and other stars
as well as much of the intergalactic matter are in the plasma phase.
Closer to home, the glowing gas in a fluorescent tube is a plasma, as
are the gases of the aurora borealis, shown in Figure 17.13.
In all phases of matter, the atoms are constantly in motion. In the
solid phase, the atoms and molecules vibrate about fixed positions.
If the rate of molecular vibration is increased enough, molecules will
shake apart and wander throughout the material, jostling in nonfixed
positions. The shape of the material is no longer fixed but takes the
shape of its container. This is the liquid phase. If more energy is put
into the material so that the molecules move about at even greater
rates, they may break away from one another and become a gas.
All substances can be transformed from one phase to another. We
often observe this changing of phase in the compound H2O. When
solid, it is ice. If we heat the ice, the increased molecular motion
jiggles the molecules out of their fixed positions, and we have liquid
water. If we heat the water, we can reach a stage where the continued
increase in molecular motion results in a separation between water
molecules, and we have steam. Continued heating causes the molecules to separate into atoms. If we heat these to temperatures
exceeding 2000°C, the atoms themselves will be shaken apart, making
a gas of ions and free electrons. Then we have a plasma.
CONCEPT
CHECK
Teaching Resources
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Workbook
• PresentationEXPRESS
• Interactive Textbook
CHAPTER 17
THE ATOMIC NATURE OF MATTER
337
337
REVIEW
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• TeacherEXPRESS
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1
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• Conceptual Physics Alive!
DVDs Atoms
Concept Summary
•
•
•
•
•
•
•
•
•
338
••••••
Every simple, complex, living, or nonliving substance is put together from a pantry
containing less than 100 elements.
Atoms are so small that there are about 1023
in a gram of water (a thimbleful).
Atoms in your body have been around since
long before the solar system came into existence, more than 4.6 billion years ago.
Brownian motion is evidence that atoms
exist, as it results from the motion of neighboring atoms and molecules. They bump
into the larger particles we can see.
Molecules can be made up of atoms of the
same elements or of different elements.
Compounds have properties different from
those of the elements of which they are
made.
The mass of an atom is primarily concentrated in the nucleus.
The arrangement of electrons in the shells
around the atomic nucleus dictates the
atom’s chemical properties.
Matter exists in four phases: solid, liquid,
gaseous, and plasma.
338
Key Terms
atoms (p. 325)
element (p. 325)
Brownian motion
(p. 328)
molecule (p. 330)
compound (p. 331)
chemical formula
(p. 331)
nucleus (p. 331)
nucleons (p. 332)
••••••
neutrons (p. 332)
protons (p. 332)
isotopes (p. 333)
atomic number
(p. 333)
ion (p. 334)
shell model of the
atom (p. 335)
periodic table
(p. 335)
plasma (p. 337)
think! Answers
17.2 Yes, and of physicist Richard Feynman too.
However, these atoms are combined differently than they were before. The next time
you feel insignificant, take comfort in the
thought that many of the atoms that compose
you will be part of the bodies of all the people
on Earth who are yet to be! In this sense, our
atoms at least, are immortal.
17.3 The mass of Earth does increase by the addition of roughly 40,000 tons of interplanetary
dust each year. But the increasing number
of people does not increase the mass of the
Earth. The atoms that make up our body
are the same atoms that were here before we
were born. The atoms that make up a baby
forming in the mother’s womb must be supplied by the food she eats. And those atoms
were formed in the stars that have long since
exploded.
ASSESS
1
Check Concepts
ASSESS
1. About 100
2. Hydrogen
3. About the same
4. From exploding stars
(supernovae)
Check Concepts
••••••
Section 17.1
9. Individual atoms cannot be seen with
visible light; yet there is an image of individual atoms in Figure 17.4. Explain.
1. Approximately how many elements are
known today?
10. What is the purpose of a model in science?
2. Which element has the lightest atoms?
Section 17.5
Section 17.2
3. How does the approximate number of
atoms in the air in your lungs compare with
the number of breaths of air in the
atmosphere of the whole world?
6. Most atoms are older.
7. Atoms migrate and spend
little time within us.
8. They are bombarded by
moving molecules.
9. The image was made using
electrons, not light.
11. Distinguish between an atom and a
molecule.
10. To make predictions
12. a. How many elements compose pure water?
b. How many individual atoms are there in a
water molecule?
13. a. Cite an example of a substance that is
made of molecules.
b. Cite a substance that is made of atoms
rather than molecules.
14. True or false: We smell things because
certain molecules are attracted to our noses.
Section 17.6
4. From where did the heaviest elements originate?
5. Atoms are much smaller.
15. a. What is a compound?
b. Cite the chemical formulas for at least
three compounds.
11. A molecule is a specific
combination of atoms.
12. a. Two; hydrogen and oxygen
b. Three; two hydrogen and
one oxygen
13. a. Water (Check students’
work for other examples.)
b. Metals
14. False; there is no attraction.
15. a. A substance made of
two or more elements
chemically combined in a
fixed proportion
b. Some examples: CO2, H2O,
NaCl (Check students’ work
for more examples.)
5. How do the sizes of atoms compare with the
wavelengths of visible light?
Section 17.3
6. How does the age of most atoms compare
with the age of the solar system?
7. What is meant by the statement that you
don’t “own” the atoms that make up your
body?
Section 17.4
8. What causes dust particles to move with
Brownian motion?
CHAPTER 17
THE ATOMIC NATURE OF MATTER
339
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16. Scattering that suggested the
existence of the nucleus
17. Nearly all the atom’s mass is
in the nucleus.
18. The nucleus is extremely small
compared to the atom.
19. Protons and neutrons
20. a. An atom with a specific
number of neutrons in the
nucleus
b. carbon-14, carbon-12
21. Same
22. Same
23. Electron’s mass is about
1/1800 of the nucleon’s mass.
24. a. Atom that has lost or
gained one or more electrons
b. Na1, Cl2 (Check students’
work for more examples.)
1
ASSESS
(continued)
Section 17.7
26. What is the periodic table of the elements?
16. What did Rutherford discover when his
group bombarded a thin foil of gold with
subatomic particles?
27. What does the atomic number of an element tell you about the element?
17. How does the mass of an atomic nucleus
compare with the mass of the whole atom?
28. According to the shell model of the atom,
how many electron shells are there in the
hydrogen atom? The lithium atom? The
aluminum atom?
18. How does the size of an atomic nucleus
compare with the size of the whole atom?
25. Atoms are themselves mostly
empty space.
Section 17.9
19. What are the two kinds of nucleons?
29. What are the four phases of matter?
26. A chart showing the elements
organized according to the
properties of their atoms
20. a. What is an isotope?
b. Give two examples of isotopes.
30. In terms of electrical conduction, how does
a plasma differ from a gas?
27. Tells you the number of
protons in its nucleus and its
place in the table
21. How does the atomic number of an element
compare with the number of protons in its
nucleus?
31. How many types of atoms can you expect to
find in a pure sample of any element?
28. 1; 2; 3
29. Solid, liquid, gas, plasma
30. Plasma conducts electricity,
gases normally don’t.
22. How does the atomic number of an element
compare with the number of electrons that
normally surround the nucleus?
Think and Explain
31. One
32. Three: two hydrogens, one
oxygen
••••••
33. Which of these formulas represent pure elements? H2, H2O, He, Na, NaCl, Au, U
Think and Explain
34. Which are older, the atoms in the body of
an elderly person, or those in a baby?
33. H2, He, Na, Au, and U are
pure elements. H2O and NaCl
are compounds.
34. No substantial difference
35. Cat leaves a trail of molecules
that mix with air and enter
nose.
32. How many individual atoms are in a water
molecule?
Section 17.8
23. How does the mass of an electron compare
with the mass of a nucleon?
35. A cat strolls across your backyard. An hour
later, a dog with his nose to the ground follows the trail of the cat. Explain this occurrence from a molecular point of view.
24. a. What is an ion?
b. Give two examples of ions.
36. Suppose you smell the shaving lotion your
brother is wearing almost immediately after
he walks into the room. From an atomic
point of view, exactly what is happening?
37. No. A body has odor only if it
sheds some of its molecules.
25. At the atomic level, a solid block of iron is
mostly empty space. Explain.
37. If no molecules in a body could escape,
would the body have any odor?
340
340
36. Molecules evaporate from
shaving lotion and mix with
molecules of the air; some get
into the nose.
38. They come from the food
that the cat eats and air it
breathes.
1
ASSESS
39. In ancient stars that long ago
exploded
(continued)
38. A kitten will add several kilograms to its
mass as it grows into a full-sized cat. From
where do the atoms that make up this added
mass originate?
39. Where were the atoms that make up a newborn baby manufactured?
40. Although you can’t see an atom through a
microscope, at some point a clump of atoms
is large enough to see as a “dot” through a
microscope. What determines when the
clump of atoms is big enough to be seen?
41. Why is Brownian motion apparent only for
microscopic particles?
42. Atoms are mostly empty space, and structures such as a floor are composed of atoms
and are therefore also mostly empty space.
Why don’t you fall through the floor?
40. Clump must be as large as
wavelength of light used to
view it.
44. If two protons and two neutrons are removed from the nucleus of an oxygen atom,
what nucleus remains?
45. What element results if you add a pair of
protons to the nucleus of mercury? (See
periodic table.)
41. Brownian motion depends on
more bumps on one side of
a particle than on the other.
The bigger the particle, the
more the bumps even out.
42. There are electrical repulsion
forces between atoms in the
floor surface and our surface.
Whirling electrons “fill up”
the space within an atom.
46. What element results if one of the neutrons
in a nitrogen nucleus is converted by radioactive decay into a proton?
43. Nitrogen (7 protons)
47. What element will result if two protons and
two neutrons are ejected from a uranium
nucleus?
46. Oxygen
48. In what way does the number of protons
in an atomic nucleus dictate the chemical
properties of the element?
49. Radon
44. Beryllium
45. Lead
47. Thorium (90 protons)
48. It dictates the number of
electrons about the nucleus.
50. It is arsenic (33 protons).
49. What element results if two protons and two
neutrons are ejected from a radium nucleus?
50. You could swallow a capsule of the element
germanium without harm. But if a proton
were added to each of the germanium
nuclei, you would not want to swallow the
capsule. Why?
43. What element will result if a proton is added
to the nucleus of carbon? (See periodic
table.)
CHAPTER 17
CHAPTER 17
THE ATOMIC NATURE OF MATTER
THE ATOMIC NATURE OF MATTER
341
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341
51. Copper, atomic number 29;
any atom with 29 protons is
copper.
52. a. Both have 27 protons.
Cobalt-59 has 32 neutrons,
and cobalt-60 has 33 neutrons.
b. 27 electrons in each
53. Almost no significance;
electron has 1/1800 mass of
proton.
54. The same; all have 82 protons.
55. Protons contribute more to
mass, electrons more to size.
56. They contain different
numbers of oxygen atoms.
Ozone molecule has 3 oxygen
atoms; oxygen molecule has
2 oxygen atoms.
57. Yes; example is oxygen gas,
O2, or ozone, O3.
58. Yes; salt (NaCl) is crystal array
of ions, not molecules.
59. When combined they form
harmless table salt.
60. Gains an electron
61. Loses an electron
62. Neon, argon, krypton, xenon,
and radon (the noble gases)
1
ASSESS
(continued)
51. A particular atom contains 29 electrons,
34 neutrons, and 29 protons. What is the
identity of this element and what is its
atomic number?
52. The atomic masses of two isotopes of
cobalt are 59 and 60.
a. What is the number of protons and
neutrons in each?
b. What is the number of orbiting electrons
in each when the isotopes are electrically
neutral?
53. When an atom loses an electron and becomes a positively charged ion, how significant is the change in the atom’s mass?
54. One isotope of lead has 82 protons and
124 neutrons in its nucleus. What can you
say about the number of protons in the
nucleus of any other isotope of lead?
55. Which contributes more to an atom’s
mass—electrons or protons? Which contributes more to an atom’s size?
56. An ozone molecule and an oxygen molecule are pure oxygen. How are they
different?
57. Is it possible to have a molecule that isn’t
a compound? Give an example.
58. Is it possible to have a compound that
isn’t made up of molecules? Give an
example.
59. If you eat metallic sodium or inhale
chlorine gas, you run a great risk of dying.
When these two elements combine, however, you can safely sprinkle the resulting
compound on your popcorn for better taste.
What is going on?
60. To become a negative ion, does an atom
lose or gain an electron?
61. To become a positive ion, does an atom
lose or gain an electron?
62. Helium is an inert gas, meaning that
it doesn’t readily combine with other elements. What five other elements would you
also expect to be inert gases? (See the
periodic table.)
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342
1
63. Table-tennis balls have lower
mass—so greater number.
Similarly, carbon has lower
mass—so greater number.
ASSESS
64. Al; less massive atoms so more
in 1 kg
(continued)
63. Why don’t equal masses of golf balls and
table-tennis balls contain the same number
of balls? Also, why don’t equal masses of
pure carbon and oxygen contain the same
number of atoms?
64. Which contains more atoms: 1 kg of
lead or 1 kg of aluminum?
65. H2; has less mass, so greater
speed for same KE
Think and Solve
66. Carbon, due to greater mass
(KE = 1/2 mv2)
••••••
70. Show that there are 16 grams of oxygen in
18 grams of water.
71. Show that there are 4 grams of hydrogen
in 16 grams of methane gas. (The chemical
formula for methane is CH4.)
72. A typical atom is around 2 × 10⫺10 m in
65. In a gaseous mixture of hydrogen and
diameter, while a baby’s hair is about
oxygen molecules, both with the same aver2 × 10⫺5 m in thickness. How many atoms
age kinetic energy, which molecules move
thick is a typical baby’s hair?
faster on average?
66. A hydrogen atom and a carbon atom
have the same speed. Which has the greater
kinetic energy?
67. Its carbon comes from CO2
absorbed from air.
68. Adding heat to a solid
changes it to liquid; add more
heat and it changes to gas;
add more and it changes
to plasma. Subtract heat
and change is in opposite
direction.
69. Open to many answers
Think and Solve
70. Twice as many H atoms
(atomic mass 1) as O atoms
(atomic mass 16)
71. For every 4 g of H, there will
be 12 g of C.
67. In what sense is it correct to say that
much of a tree is solidified air?
68. The phases of matter are solid, liquid,
gas, and plasma. What does the addition or 73. A typical atom is around 2 × 10⫺10 m in
subtraction of heat have to do with changes
diameter, while a typical bacterium is about
of phase?
10⫺6 m in diameter. How many atoms thick is
the typical bacterium?
69. Write a letter to your grandparents that
discusses the importance of the periodic
74. Gas A is composed of diatomic molecules
table of elements. Also tell them what you’ve
(two atoms to a molecule) of a pure element.
learned about the differences among atoms,
Gas B is composed of monatomic molecules
elements, and molecules.
(one atom to a molecule) of another pure
72. Hair is (2 3 1025 m/
2 3 10210 m) 5 105 atoms
thick—about 100,000 atoms
thick.
73. Bacterium is about (1026 m/
2 3 10210 m) 5 about 5,000
atoms thick.
74. Gas A has three times the mass
of Gas B. There are twice as
many atoms in A, so the mass
of each atom must be half of
three times as much, 3/2.
element. Gas A has three times the mass of an
equal volume of gas B at the same temperature and pressure. How do the atomic masses
of elements A and B compare?
Teaching Resources
• Computer Test Bank
• Chapter and Unit Tests
CHAPTER 17
CHAPTER 17
THE ATOMIC NATURE OF MATTER
THE ATOMIC NATURE OF MATTER
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