Table of Contents
17
Unit 4: The Nature of Matter
Chapter 17: Properties of Atoms and the
Periodic Table
17.1: Structure of the Atom
17.2: Masses of Atoms
17.3: The Periodic Table
Structure of the Atom
17.1
Scientific Shorthand
• Scientists have
developed their
own shorthand for
dealing with long,
complicated names.
• Chemical symbols
consist of one capital
letter or a capital
letter plus one or two
smaller letters.
Structure of the Atom
17.1
Scientific Shorthand
• For some elements, the symbol is the first
letter of the element's name.
• For other elements, the symbol is the first
letter of the name plus another letter from its
name.
• Because scientists worldwide use this system,
everyone understands what the symbols
mean.
Structure of the Atom
17.1
Atomic Components
• An element is matter that is composed of one
type of atom, which is the smallest piece of
matter that still retains the property of the
element.
• Atoms are composed of
particles called protons,
neutrons, and electrons.
Click image to view movie
Structure of the Atom
17.1
Atomic Components
• Protons and neutrons are found in a small
positively charged center of the atom called
the nucleus that is surrounded by a cloud
containing electrons.
• Protons are
particles
with an
electrical
charge of
1+.
Structure of the Atom
17.1
Atomic Components
• Electrons are particles with an electrical
charge of 1–.
• Neutrons are neutral particles that do not
have an electrical charge.
Structure of the Atom
17.1
Quarks—Even Smaller Particles
• Protons and neutrons are made up of smaller
particles called quarks.
• So far, scientists have confirmed the
existence of six uniquely different quarks.
Structure of the Atom
17.1
Quarks—Even Smaller Particles
• Scientists theorize that an arrangement of
three quarks held together with the strong
nuclear force produces a proton.
• Another arrangement of three quarks
produces a neutron
Structure of the Atom
17.1
Finding Quarks
• To study quarks, scientists accelerate charge
particles to tremendous speeds and then force
them to collide with—or smash into—
protons. This collision causes the proton to
break apart.
• The particles that result from the collision
can be detected by various collection devises.
Structure of the Atom
17.1
Models—Tools for Scientists
• Scientists and engineers use models to
represent things that are difficult to
visualize—or picture in your mind.
• Scaled-down models allow you to see either
something too large to see all at once, or
something that has not been built yet.
• Scaled-up models are often used to visualize
things that are too small to see.
“Atomic Structure”
Studying Atoms
• Different Models of the atom
– Greek Model
– Dalton’s Atomic Theory
– Thomson’s Model
– Rutherford’s Atomic Theory
– Bohr’s Model of the Atom
– Electron Cloud Model
Structure of the Atom
17.1
Models—Tools for Scientists
• To study the atom, scientists have developed
scaled-up models that they can use to
visualize how the atom is constructed.
• For the model to be useful, it must support all
of the information that is known about matter
and the behavior of atoms.
Aristotle
• Aristotle thought that all substances were
built up from only 4 elements
– Earth
– Air
– Fire
– Water
These elements were a
combination of 4 qualities
hot, cold, dry, and wet
The Hellenic Market
Fire
~
~
Water
Earth
Air
Aristotle
Aristotle
• Aristotle did not think there was a
limit to the number of times matter
could be divided.
»For centuries people
believed this model.
Defining the Atom
• The Greek philosopher Democritus (460
B.C. – 370 B.C.) was among the first to
suggest the existence of atoms (from the
Greek word “atomos”) ATOM
– He believed that atoms were indivisible and
indestructible
– His ideas did agree with later scientific theory,
but did not explain chemical behavior, and
was not based on the scientific method–
but just philosophy
Greek Model
“To understand the very large,
we must understand the very small.”
Democritus
• Greek philosopher
• Idea of ‘democracy’
• Idea of ‘atomos’
– Atomos = ‘indivisible’
– ‘Atom’ is derived
• No experiments to support
Democritus’s model of atom
idea
No protons, electrons, or neutrons
• Continuous vs. discontinuous
Solid and INDESTRUCTABLE
theory of matter
Who Was Right?
•
•
•
•
•
•
•
•
California WEB
Greek society was slave based
Beneath famous to work with hands
did not experiment
Greeks settled disagreements by
argument
Aristotle was more famous
He won!
His ideas carried through middle ages.
Alchemists change lead to gold
John Dalton
John Dalton
• John Dalton (17661844) was known as the
“Father of the
modern atom.”
• He was the first to actually
test, previously the Greeks
would just theorize. In his
work he developed
Dalton's Atomic Theory.
Structure of the Atom
17.1
The Changing Atomic Model
• In the 1800s, John Dalton, an English
scientist, was able to offer proof that
atoms exist.
Because Dalton’s atomic theory was
proven through many experiments 
His THEORY became widely accepted.
Dalton’s Atomic Theory (experiment based)
1) All elements are composed of tiny
indivisible particles called atoms
2) Atoms of the same element are
identical.
3) Atoms Can’t be Created nor
Destroyed!
John Dalton
(1766 – 1844)
4) Atoms of different elements combine in simple
whole-number ratios to form chemical compounds
5) In a particular compound, atoms of different
elements always combine in the same way
6) All atoms of the same element have the same mass,
atoms of diff. elements have diff. masses
Dalton’s Model
• The Elements are pictured as solid
spheres:
• Each type of atom is represented by a
tiny, solid sphere with a different mass.
Dalton’s Elements
John Dalton
1808
JJ Thomson
JJ. Thomson
• J.J. used the idea behind charges:
-- Like charges repel
-- Unlike charges attract
With this idea in mind, J.J. used electric
current to study the atom.
 Remember my ex: with the Magnets
Sir William Crookes (1832 - 1919) was the British
scientist who invented the cathode ray tube. His work
paved the way to the discovery of the electron.
-
voltage
source
William Crookes
+
vacuum tube
magnet
metal disks
Crooke’s Tube
Discovery of the Electron
Looking at what JJ saw, what do you think he
determined was the charge on the particles in the
beam?
Thomson’s Experiment
ON
-
OFF
voltage
source
By adding an electric field…
+
+
-
he found that the moving pieces were negative.
In 1897, J.J. Thomson used a
cathode ray tube to deduce the
presence of a negatively charged
particle: the electron
 The Electron was named by G. Johnstone
Stoney !
** Thomsons experiments were the
1st proof that atoms are made of
smaller particles.
Mass of the Electron
Mass of the
electron is
9.11 x 10-28 g
The oil drop apparatus
1916 – Robert Millikan determines the mass
of the electron: 1/1840 the mass of a
hydrogen atom; has one unit of negative
charge
Conclusions from the Study of the
Electron:
a) Cathode rays have identical properties
regardless of the element used to
produce them. All elements must contain
identically charged electrons.
b) Atoms are neutral, so there must be
positive particles in the atom to balance
the negative charge of the electrons
c) Electrons have so little mass that atoms
must contain other particles that account
for most of the mass
Some Modern Cathode Ray Tubes
Cathode ray tubes pass electricity
through a gas that is contained at a very
low pressure.
J.J. Thomson
• J.J. reasoned that if the atom had
negative little particles inside it, but
was considered to be NEUTRAL,
then there must also be some positive
particles too.
Thomson’s Atomic Model
J. J. Thomson
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum pudding”
model.
Conclusions from the Study of the
Electron:
 Eugen Goldstein in 1886 observed
what is now called the “proton” particles with a positive charge, and
a relative mass of 1 (or 1840 times
that of an electron)
 1932 – James Chadwick confirmed
the existence of the “neutron” – a
particle with no charge, but a mass
nearly equal to a proton
Rutherford’s
Atomic
Theory
Ernest Rutherford (1871-1937)
• Learned physics in J.J.
Thomson’ lab.
• Noticed that ‘alpha’
particles were
sometime deflected by
something in the air.
• Gold-foil experiment
Ernest Rutherford’s
Gold Foil Experiment - 1911
 Alpha particles are helium nuclei - The alpha
particles were fired at a thin sheet of gold foil
 Particle that hit on the detecting screen (film) are
recorded
Rutherford ‘Scattering’
1.
2.
3.
In 1909 Rutherford undertook a series of experiments
He fired a (alpha) particles at a very thin sample of gold foil
According to the Thomson model the a particles should only
be slightly deflected
4.
Rutherford discovered that they were deflected
through large angles and could even be reflected
straight back to the source
Lead collimator
Gold foil
a particle
source
q
Rutherford’s problem:
In the following pictures, there is a target
hidden by a cloud. To figure out the shape of
the target, we shot some beams into the cloud
and recorded where the beams came out. Can
you figure out the shape of the target?
Target
#1
Target
#2
The Answers:
Target #1
Target #2
Rutherford’s Findings
Most of the particles passed right through
 A few particles were deflected
 VERY FEW were greatly deflected

“Like howitzer shells bouncing
off of tissue paper!”
Conclusions:
a) The nucleus is small
b) The nucleus is dense
c) The nucleus is positively
charged
152 mm
howitzer-gun M1937
The Rutherford Atomic Model
Based on his experimental evidence:
The atom is mostly empty space
All the positive charge, and almost all
the mass is concentrated in a small area
in the center. He called this a
“nucleus”
The Rutherford Atomic Model
• The nucleus is composed of
protons and neutrons
–The electrons distributed around
the nucleus, and occupy most of
the volume
–His model was called a “nuclear
model”
Bohr’s Model
of the Atom
Niels Bohr
• In the Bohr Model (1913)
the neutrons and protons
occupy a dense central
region called the nucleus,
and the electrons orbit
the nucleus much like
planets orbiting the Sun.
• They are not confined to
a planar orbit like the
planets are.
Bohr Model
Planetary
model
After Rutherford’s discovery, Bohr proposed that
electrons travel in definite orbits at constant
speeds around the nucleus like planets around
the sun.
Staircase Idea
• To understand energy levels, picture them
as steps in a staircase.
• Just as you can’t stand on 2 steps at one
time, Electrons can’t occupy more than 1
energy level at a time.
• Bottom step = Lowest Energy Level
• Top Step = Highest Energy Level
• Cont…
Staircase Model
• An electron in an ATOM can
move from one energy level to
another if enough energy is
provided.
• The size of the jump ( 1 level
or 2) determines the amt. of
energy gained or lost.
Humor
• Two atoms are walking down the street.
• One atom says to the other, “Hey! I think I lost an
electron!”
• The other says, “Are you sure??”
• “Yes, I’m positive!”
•
•
•
•
A neutron walks into a restaurant and orders a couple
of drinks. As she is about to leave, she asks the waiter
how much she owes. The waiter replies, “For you,
No Charge!!!”
Structure of the Atom
17.1
The Electron Cloud Model
• By 1926, scientists had developed the
electron cloud model of the atom that is
in use today.
• An electron
cloud is the area
around the
nucleus of an
atom where its
electrons are most
likely found.
Structure of the Atom
17.1
The Electron Cloud Model
• The electron cloud is 100,000 times larger
than the diameter of the nucleus.
• In contrast, each electron in the cloud is
much smaller than a single proton.
• Because an electron's mass is small and the
electron is moving so quickly around the
nucleus, it is impossible to describe its exact
location in an atom.
• EX: Propeller of an Airplane’s wings !
Section Check
17.1
Question 1
Which is the smallest piece of matter that
still retains the property of the element?
A. atom
B. quark
C. neutron
D. proton
Section Check
17.1
Question 2
What particles are found in the nucleus of an
atom?
A. protons and electrons
B. protons and neutrons
C. neutrons and electrons
D. quarks and electrons
Section Check
17.1
Question 3
What is the name of the small particles that
make up protons and neutrons?
Question 4
• The Greek philosopher Democritus coined
what word for a tiny piece of matter that
cannot be divided?
• a. element c. electron
• b. atom
d. molecule
Question 5
Which of the following is NOT part of
John Dalton’s atomic theory?
• a. All elements are composed of atoms.
• b. All atoms of the same element have the
same mass.
• c. Atoms contain subatomic particles.
• d. A compound contains atoms of more
than one element.
Question 6
Which of the following most accurately represents
John Dalton’s model of the atom?
• a. a tiny, solid sphere with an unpredictable
mass for a given element
• b. a hollow sphere with a dense nucleus
• c. a tiny, solid sphere with predictable mass for
a given element
• d. a sphere that is hollow throughout
Question 7
• JJ Thomson’s experiments provided
evidence that an atom
• a. is the smallest particle of matter.
• b. contains negatively charged particles.
• c. has a negative charge.
• d. has a positive charge.
Question 8
Rutherford’s gold foil experiment provided
evidence for which of the following statements?
• a. Negative and positive charges are spread
evenly throughout an atom.
• b. Alpha particles have a positive charge.
• c. Gold is not as dense as previously thought.
• d. There is a dense, positively charged mass in
the center of an atom.
Question 9
• Who provided evidence for the existence
of a nucleus in an atom?
• a. John Dalton c. Democritus
• b. J.J. Thomson d. Ernest Rutherford
Question 10
In an atomic model that includes a
nucleus, positive charge is
• a. concentrated in the center of an atom
• b. spread evenly throughout an atom.
• c. concentrated at multiple sites in an
atom.
• d. located in the space outside the
nucleus.
Part 2
Masses of Atoms
17.2
Atomic Mass
• The nucleus contains
most of the mass of
the atom because
protons and neutrons
are far more massive
than electrons.
• The mass of a proton
is about the same as
that of a neutron—
approximately
Masses of Atoms
17.2
Atomic Mass
• The mass of
each is
approximately
1,836 times
greater than the
mass of the
electron.
Masses of Atoms
17.2
Atomic Mass
• The unit of measurement used for atomic
particles is the ______________________.
atomic mass unit (amu)
• The mass of a proton or a neutron is almost
equal to 1 amu.
• The atomic mass unit is defined as onetwelfth the mass of a carbon atom containing
six protons and six neutrons.
Masses of Atoms
17.2
Protons Identify the Element
• The number of protons tells you what type
of atom you have and vice versa. For
example, every carbon atom has six protons.
Also, all atoms with six protons are carbon
atoms.
• The number of protons in an atom is equal to
a number called the atomic number.
Masses of Atoms
17.2
Mass Number
• The mass number of an atom is the sum of
the number of protons and the number of
neutrons in the nucleus of an atom.
Masses of Atoms
17.2
Mass Number
• If you know the mass number and the atomic
number of an atom, you can calculate the
number of neutrons.
number of neutrons = mass number – atomic number
Masses of Atoms
17.2
Isotopes
• Not all the atoms of an element
have the same number of
neutrons.
• Atoms of the same element that
have different numbers of neutrons
are called isotopes.
• Remember – Protons Never Change
Masses of Atoms
17.2
Identifying Isotopes
• Models of two isotopes of boron are shown.
Because the numbers of neutrons in the
isotopes are different, the mass numbers are
also different.
• You use the name of
the element followed
by the mass number of
the isotope to identify
each isotope: boron10 and boron-11.
Masses of Atoms
17.2
Identifying Isotopes
• The average atomic mass of an element is
the weighted-average mass of the mixture of
its isotopes.
• For example, four out of five atoms of boron
are boron-11, and one out of five is boron-10.
• To find the weighted-average or the average
atomic mass of boron, you would solve the
following equation:
Plug the following in
on a calculator
Order:
1. 4 / 5 X 11 = ?
2. 1 / 5 X 10 = ?
3. Add the 2 numbers = ?
Section Check
17.2
Question 1
How is the atomic number of an element
determined?
Section Check
17.2
Question 2
The element helium has a mass number of 4
and atomic number of 2. How many
neutrons are in the nucleus of a helium
atom?
Section Check
17.2
Question 3
How much of the mass of an atom is
contained in an electron and what is the
charge of an electron?
Part 3
Masses of Atoms
17.2
Atomic Mass
The Periodic Table
17.3
Organizing the Elements
• Periodic means "repeated in a pattern."
• In the late 1800s, Dmitri Mendeleev, a
Russian chemist, searched for a way to
organize the elements.
• When he arranged all the elements known
at that time in order of increasing atomic
masses, he discovered a pattern.
The Periodic Table
17.3
Organizing the Elements
• Because the pattern repeated, it was
considered to be periodic. Today, this
arrangement is called a periodic table of
elements.
• In the periodic table, the elements are
arranged by increasing atomic number and by
changes in physical and chemical properties.
The Periodic Table
17.3
Mendeleev's Predictions
• Mendeleev had to leave blank spaces in his
periodic table to keep the elements properly
lined up according to their chemical
properties.
• He looked at the properties and atomic
masses of the elements surrounding these
blank spaces.
The Periodic Table
17.3
Mendeleev's Predictions
• From this
information, he
was able to
predict the
properties and
the mass
numbers of new
elements that
had not yet been
discovered.
The Periodic Table
17.3
Mendeleev's Predictions
• This table shows
Mendeleev's
predicted
properties for
germanium,
which he called
ekasilicon. His
predictions
proved to be
accurate.
The Periodic Table
17.3
Improving the Periodic Table
• On Mendeleev's table, the atomic mass
gradually increased from left to right. If you
look at the modern periodic table, you will
see several examples, such as cobalt and
nickel, where the mass decreases from left to
right.
The Periodic Table
17.3
Improving the Periodic Table
• In 1913, the work of Henry G.J. Moseley, a
young English scientist, led to the
arrangement of elements based on their
increasing atomic numbers instead of an
arrangement based on atomic masses.
• The current periodic table uses Moseley's
arrangement of the elements.
The Periodic Table
17.3
The Atom and the Periodic Table
• The vertical columns in the periodic table are
called groups, or families, and are numbered
1 through 18.
• Elements in each group have similar
properties.
The Periodic Table
17.3
Electron Cloud Structure
• In a neutral atom, the number of electrons is
equal to the number of protons.
• Therefore, a carbon atom, with an atomic
number of six, has six protons and six
electrons.
Bohr Model Practice
• In atomic physics, the Bohr model,
devised by Niels Bohr, depicts the atom as
a small, positively charged nucleus
surrounded by electrons that travel in
circular orbits around the nucleus—similar
in structure to the solar system, but with
electrostatic forces providing attraction,
rather than gravity
Examples of Bohr Models
What Element am I ?
The Periodic Table
17.3
Electron Cloud Structure
• Scientists have found that electrons within
the electron cloud have different amounts of
energy.
The Periodic Table
17.3
Electron Cloud Structure
• Scientists model the energy differences of the
electrons by placing the electrons in energy
levels.
The Periodic Table
17.3
Electron Cloud Structure
• Energy levels nearer the nucleus have lower
energy than those levels that are farther away.
• Electrons fill these energy levels from the
inner levels (closer to the nucleus) to the
outer levels (farther from the nucleus).
The Periodic Table
17.3
Electron Cloud Structure
• Elements that are in the same group have the
same number of electrons in their outer
energy level.
• It is the Number of electrons in the outer
energy level that determines the chemical
properties of the element.
The Periodic Table
17.3
Energy Levels
• The maximum number of electrons that can
be contained in each of the first four levels is
shown.
The Periodic Table
17.3
Energy Levels
• For example, energy level one can contain a
maximum of two electrons.
• A complete and stable outer energy level will
contain eight electrons.
The Periodic Table
17.3
Rows on the Table
• Remember that the atomic number found on
the periodic table is equal to the number of
electrons in an atom.
The Periodic Table
17.3
Rows on the Table
• The first row has hydrogen with one electron
and helium with two electrons both in energy
level one.
• Energy level one can hold only two electrons.
Therefore, helium has a full or complete
outer energy level.
The Periodic Table
17.3
Rows on the Table
• The second row begins with lithium, which
has three electrons—two in energy level one
and one in energy level two.
• Lithium is followed by beryllium with two
outer electrons, boron with three, and so on
until you reach neon with eight outer electrons.
The Periodic Table
17.3
Rows on the Table
• Do you notice how the row in the periodic
table ends when an outer level is filled?
• In the third row of elements, the electrons
begin filling energy level three.
• The row ends with argon, which has a full
outer energy level of eight electrons.
The Periodic Table
17.3
Electron Dot Diagrams
• Elements that are in the same group have the
same number of electrons in their outer
energy level.
• These outer electrons are so important in
determining the chemical properties of an
element that a special way to represent them
has been developed.
The Periodic Table
17.3
Electron Dot Diagrams
• An electron dot diagram uses
the symbol of the element and
dots to represent the electrons in
the outer energy level.
• Electron dot diagrams are used
also to show how the electrons in
the outer energy level are bonded
when elements combine to form
compounds.
The Periodic Table
17.3
Same Group—Similar Properties
• The elements in
Group 17, the
halogens, have
electron dot
diagrams similar
to chlorine.
• All halogens have
seven electrons in
their outer energy
levels.
The Periodic Table
17.3
Same Group—Similar Properties
• A common property of the halogens is the
ability to form compounds readily with
elements in Group 1.
• The Group 1 element, sodium, reacts easily
with the Group 17 element, chlorine.
• The result is the
compound sodium
chloride, or
NaCl—ordinary
table salt.
The Periodic Table
17.3
Same Group—Similar Properties
• Not all elements will combine readily with
other elements.
• The elements in
Group 18 have
complete outer
energy levels.
• This special
configuration makes
Group 18 elements
relatively unreactive.
The Periodic Table
17.3
Regions on the Periodic Table
• The periodic table has several regions with
specific names.
• The horizontal rows of elements on the
periodic table are called periods.
• The elements increase by one proton and one
electron as you go from left to right in a
period.
The Periodic Table
17.3
Regions on the Periodic Table
• All of the elements in the blue squares are
metals.
The Periodic Table
17.3
Regions on the Periodic Table
• Those elements on the right side of the periodic
table, in yellow, are classified as nonmetals.
The Periodic Table
17.3
Regions on the Periodic Table
• The elements in green are metalloids or
semimetals.
The Periodic Table
17.3
A Growing Family
• In 1994, scientists at the Heavy-Ion Research
Laboratory in Darmstadt, Germany,
discovered element 111.
• Element 112 was discovered at the same
laboratory.
• Both of these elements are produced in the
laboratory by joining smaller atoms into a
single atom.
The Periodic Table
17.3
Elements in the Universe
• Using the technology that is available today,
scientists are finding the same elements
throughout the universe.
• Many scientists believe that hydrogen and
helium are the building blocks of other
elements.
The Periodic Table
17.3
Elements in the Universe
• Exploding stars, or supernovas, give
scientists evidence to support this theory.
• Many scientists believe that supernovas have
spread the elements that are found throughout
the universe.
Section Check
17.3
Question 1
How are the elements arranged in the
periodic table?
Section Check
17.3
Question 2
What do the elements in a vertical column of
the periodic table have in common?
Section Check
17.3
Question 3
What do the dots in this electron dot diagram
represent?