Chapter 7
Atoms, Elements,
Compounds and Mixtures
7.1 Structure of an Atom
• OBJECTIVES:
• Summarize how models of the atom have
changed.
• Name and describe parts of an atom
• Calculate the number of protons, neutrons
and electrons in an atom given its mass
number.
Structure of an Atom
• YOU have learned that all matter in the
universe is made up of tiny particles.
• A glass of water has many water
particles, each too small to see.
• If you could see them, what would they
look like?
• What makes them different from the
particles of other kinds of matter?
Atoms and Elements
• Water particles can actually be divided
into even smaller pieces of matter.
• These particles are examples of the most
basic units of matter called atoms.
• Atoms can't be broken down into smaller
pieces by any common methods of
separating matter.
Atoms and Elements
• Atoms are the building blocks of the
universe.
• Scientists have identified nearly 100
different kinds of naturally occurring
atoms.
• Each kind of atom has unique properties and
is called an element.
• An atom of an element can't be broken
down and retain its properties.
Atoms and Elements
• The particles of matter can be
made of single atoms, two or
more atoms of the same
element, or two or more atoms
of different elements.
• This water particle is made up
of three atoms, two of
hydrogen and one of oxygen.
Models of the Atom
• All atoms share the same basic structure.
• During the past 200 years, scientists have
proposed different models for this
structure.
• Each model was the best one for its time.
• With new observations or experiments,
however, the model had to be changed.
• http://highered.mcgrawhill.com/sites/dl/free/0078600499/164155/00044672.html
Dalton's Model
•
* In the early 1800s, John Dalton
performed experiments with gases.
• His results convinced him that matter was
made up of tiny particles.
• …and that each element must be made of its
own unique kind of particle and that these
particles combine in simple ways.
Dalton's Model
• Dalton called these basic particles atoms and
pictured them as tiny, solid spheres.
Dalton's Model
Based on his experiments, Dalton developed a
theory of the structure of matter. His
theory contained four main concepts:
1. All matter is composed of tiny, indivisible
particles called atoms.
2. Atoms of each element are exactly alike.
3. Atoms of different elements have different
masses.
4. Atoms of different elements can join to form
compounds.
Thomson's Model
• At the end of the 1800s, J. J. Thomson
discovered that atoms were not just simple,
solid spheres and contained even smaller,
subatomic particles.
• The subatomic particles Thomson discovered
were very small and negatively charged.
• Thompson called them electrons.
Thomson's Model
• Thomson knew that atoms are electrically
neutral. Therefore, he reasoned, an atom must
contain enough positive charge to balance the
negative charge of the electrons.
• Thomson developed an atomic model in which
electrons were stuck into a positively charged
sphere.
• A positive charge in the substance of the
sphere balanced the electrons' charge .
Rutherford's Model
• By the early 1900s, scientists knew that the
positive charge of an atom comes from
subatomic particles called protons.
• A proton is a positive particle with a mass
much greater than that of an electron.
• At that time, scientists hypothesized that
electrons and protons were evenly scattered
throughout an atom.
Rutherford's Model
• In 1911 , Ernest Rutherford set out to test this
theory.
• After experimenting with a beam of positively
charged light, he concluded that the protons are
concentrated in a small area at the center of the
atom.
• He called this region the nucleus.
Rutherford's Model
• In his model, an atom is mostly empty
space. The nucleus is tiny compared to the
whole atom, but it contains nearly all the
atom's mass.
• http://www.mhhe.com/physsci/chemistry/e
ssentialchemistry/flash/ruther14.swf
Rutherford's Model
Bohr's Model
Niels Bohr modified Rutherford's model in
1913. He proposed that each electron in an atom
has a fixed amount of energy.
• This energy keeps an electron moving around the
nucleus within a specific region called an energy
level.
• In Bohr's model, energy levels surround the
nucleus in rings or shells, like the layers of an
onion.
•
Bohr's Model
In the Bohr Model, an electron can
move from one energy level to another just as
you can climb up or down a ladder, by absorbing or
releasing a specific amount of energy.
• Just as you can't be between rungs on a ladder,
an electron can't be between energy levels.
• Bohr's model has been called the ‘planetary
model’ as it compares electrons to planets and
the nucleus to the sun.
•
Bohr's Model
•
Electron Cloud Model
• Today scientists know that electrons do not actually orbit
the nucleus as in Bohr's planetary model.
• The electron cloud model is now used to describe atoms.
• In this model, electrons dart about within an energy level
in an ever-changing path.
• Most of this path falls into a region called an electron
cloud.
• At any given time, there is a high probability that the
electron exists in the electron cloud.
Electron Cloud Model
http://www.youtube.com/watch?v=zYeRSgiypbc
• The idea of an electron cloud is not so strange.
You have probably seen the blur of a fan when it
spins at high speed.
• The fast-moving blades appear to fill the space
between them, just as fast-moving electrons
seem to fill the space around the nucleus.
• The paths of an atom's electrons account for
nearly all of its volume.
•
Inside the Nucleus
As scientists learned more about atomic
structure, they found that the nucleus is
more complicated than they had thought.
• In 1932, scientists showed that most atomic nuclei
contain a third kind of subatomic particle, called a
neutron.
• A neutron has about the same mass as a proton but
has no electrical charge.
• An atomic nucleus is a positively charged, tightlypacked cluster of protons and neutrons.
Inside the Nucleus
Atomic Numbers and Isotopes
• All atoms of an element contain the same number of
protons.
• This number, called the atomic number, identifies an
element.
• Sodium, for example, has an atomic number of 11.
Atomic Numbers and Isotopes
• If all iron atoms contain 26 protons, what is the
atomic number of iron?
• The atomic number also represents the number of
electrons in an atom.
• Remember that an atom is electrically neutral. Thus
the number of negative particles must equal the number
of positive particles.
Atomic Numbers and Isotopes
• As you may recall, Dalton hypothesized that all
atoms of an element are exactly alike. Today
scientists know that he was not completely
correct.
• Atoms of the same element do have the same
number of protons and electrons, but they may
differ in the number of neutrons they contain.
• Atoms of the same element with different
numbers of neutrons are called isotopes.
Atomic Numbers and Isotopes
Mass Number and Atomic Mass
• Almost every atom contains one or more
neutrons in it’s nucleus.
• The number of neutrons does not affect the
charge of an atom, but it does affect its mass.
• The total number of protons and neutrons in an
atom is called its mass number.
• The mass number helps to distinguish one
isotope from another.
• What is the difference between carbon-12 and
carbon-14?
Brain Pop!
http://www.brainpop.com/s
cience/matterandchemistry
/isotopes/preview.weml
Check & Explain pg. 163
Answer Question 4
Draw one of each model
of a neon atom.
Dalton, Thomson,
Rutherford and Bohr
7.2 Elements
Objectives
• Describe what elements are.
• Give examples of common elements.
• Make a model that relates the
particle model to a familiar property
of matter.
Elements
• High in the Rocky Mountains or on
the streets of New York City, the
oxygen in the air you breathe is the
same.
• The air is different, but the oxygen
atoms in both places have the same
properties. All oxygen atoms have
eight protons and eight electrons.
Elements
• Oxygen is an element.
• As you have learned, elements are
the basic kinds of matter in the
universe.
• An element is a substance made of
just one kind of atom.
• It cannot be broken down or changed
by chemical means.
Elements and Matter
• Even though all matter is made up of elements,
only a few elements exist in nature in their pure
form.
• You may remember that the oxygen and nitrogen
in air are elements.
• Diamond is a natural form of the element carbon.
• Occasionally pure deposits of silver, gold, or
copper are found.
Elements and Matter
• You would probably not recognize most
elements in their pure form because
most elements in nature are combined
with other elements.
Elements and Matter
• Of the more than 100 known chemical elements,
only about 30 play an important role in your daily
life.
• About 18 elements do not occur in nature. They
are created in laboratories and known as
synthetic elements.
Properties of Elements
• The properties used to describe the elements in
their pure forms include luster, texture, color,
density and the ability to conduct electricity.
• Elements differ in how they react with other
elements. Most elements are solids but some are
gases and others are liquids.
Chemical Symbols
• Chemists use symbols to represent the names
of elements (and atoms).
• A chemical symbol is one or two letters taken
from the name of the element.
• In some cases, the symbol is derived from the
element's name in a language other than
English.
• The symbol for gold, Au, comes from aurum,
the Latin word for gold.
Chemical Symbols
• Symbols form a kind of universal
chemical shorthand. Ca means calcium
to chemists in Japan, Mexico, Kenya,
India, and everywhere else.
Chemical Symbols
Page 167
Check & Explain pg. 167
Answer questions 1 & 2
Page 167
7.3 Compounds
Objectives
• Describe the properties of a compound.
• Give examples of common compounds.
• Define operationally the composition of a
compound by writing its chemical formula.
Compounds
• Nearly all the products you use are made of
more than one element.
• The clothes you wear, the food you eat for
breakfast, and the toothpaste you use to
brush your teeth are all combinations of
elements.
• Almost everything you can think of is made
up of some combination of elements.
Defining Compounds
• There are millions of compounds in, on, and
around the earth. Many compounds are
found in living things.
• Compounds also make up most of the
nonliving world.
• Water is a compound.
Defining Compounds
• How are compounds formed?
• Many are created by geologic processes deep in the
earth.
• Organisms must manufacture compounds to stay alive.
Plants are always making the compound glucose.
• Many products you buy are made of compounds that
aren't found in the natural world.
• For example, if you look at the list of ingredients on food
packages you will probably see some compounds you don't
recognize. People create these compounds in factories and
chemical plants.
Properties of Compounds
• The properties of a compound are different from those
of the elements that make it up.
• Hydrogen and oxygen are both gases. but they combine to
form water.
• The elements that make it up always combine in a specific
proportion.
• Carbon dioxide is two parts oxygen and one part carbon
(CO2).
Properties of Compounds
Study the text and photographs on pg. 169
Answer on a separate piece of paper:
-What is the difference between the compounds carbon
dioxide and carbon monoxide?
-How does the compound sodium chloride differ from the
elements sodium and chloride?
-What compound is common in rocks, and what elements
does the compound contain?
-What is the natural form of calcium carbonate, and what
three elements make it up?
Properties of Compounds
Study the text and photographs on pg. 169
Answer on a separate piece of paper:
-CO2 has 2 atoms of oxygen and 1 atom of carbon and CO
has one C & one O.
-NaCl2 is table salt, an edible crystal. Sodium is a silvery
metal and chlorine is a poisonous gas.
-Silicon Dioxide; silicon and oxygen.
-Limestone; Calcium, carbon and oxygen.
Types of Compounds
• Compounds differ in the kinds of atoms that
make them up. Compounds also differ in the way
the atoms are joined.
• Compounds can be classified into two groups
based on how their atoms are bonded, or joined
to each other.
• 1. Molecular Compounds
• 2. Ionic Compounds
Molecular Compounds
• A molecule is a particle of matter made up of two
or more atoms held together by the sharing of
electrons.
• A compound made up of molecules is a molecular
compound.
• Although many compounds are molecules, some
molecules are not compounds.
• Oxygen gas, for example, exists
as molecules formed by two oxygen
atoms.
Ionic Compounds
• An ionic compound is a combination of positive and
negative ions, not really called atoms.
• The ions are held together by electrical attraction.
• Most ionic compounds are solids. In ionic compounds
that are solids, the ions are arranged in a regular
three-dimensional crystal structure.
• Ionic compounds are usually soluble in water. When
melted or dissolved in water, they conduct
electricity.
Ionic Compounds
• http://www.youtube.com/watch?v=LRVW0tgSLRI
Formulas of Compounds
• A chemical formula is a combination of symbols and
numbers that represent the composition of a compound.
• The symbols show the kinds of atoms in the compound.
The numbers, called subscripts, show the number of each
kind of atom.
• When more than one atom of an element is present in a
compound, a subscript is written to the right and below
the element's symbol.
• If there is only one atom of the element, no subscript is
used.
Formulas of Compounds
• How do you write a formula for a
compound?
• First you need to know what it made of. A
molecule of carbon dioxide, for example,
has one atom of carbon and two atoms of
oxygen.
• The formula is CO2.
Formulas of Compounds
7.4 Mixtures
Objectives
• Compare and contrast mixtures and
compounds.
• Distinguish between homogeneous and
heterogeneous mixtures.
• Define operationally a method for
separating a mixture.
Comparing Mixtures and Compounds
• How is a mixture different from a
compound?
• The different parts of a compound -either
atoms or ions- are combined chemically.
• The different parts of a mixture, in
contrast, are simply mixed
together.
Comparing Mixtures and Compounds
Also, mixtures and compounds differ in other ways:
1. The makeup, or composition, of a mixture can
vary. The composition of a compound, on the
other hand, is constant.
2. The components of a mixture keep their original
properties. You can still taste the sodium
chloride dissolved in sea water. A compound,
however, has properties different from the
elements that make it up.
Comparing Mixtures and Compounds
3. Because the components of a mixture are not
combined chemically, they can usually be
separated by physical means. Distillation and
filtering are examples of physical means used
to separate mixtures.
4. In contrast, the elements in a compound must
usually be separated by chemical means, such
as the addition of heat energy.
http://chrome.brainpop.com/science/matterandche
mistry/compoundsandmixtures/preview.weml
Types of Mixtures
There are several types of
mixtures:
1.Homogenous
2.Heterogeneous
3.Alloys
Homogenous Mixtures
• All parts of a homogeneous mixture
contain the same amount of each
component.
• Most mixtures formed by
dissolving a compound in a liquid are
homogeneous mixtures.
• Perfume is a homogeneous mixture
of dozens of fragrance compounds
dissolved in alcohol.
Heterogeneous Mixtures
• Not every part of a heterogeneous
mixture has the same composition.
• One part of the mixture has more
of one component than another.
• Cooking spices are often a
heterogeneous mixture of many
herbs mixed together.
Alloys
• Different metals can be combined to form a
special kind of mixture, called an alloy.
• An alloy is made by heating two or more metals
until they melt together.
• All coins are made of alloys.
• Is an alloy a homogeneous
or a heterogeneous mixture?
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