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Chapter 4- Atomic Structure
Studying the structure of atoms
is a little like studying wind.
Because you cannot see air, you
must use indirect evidence to
tell the direction of the wind.
Atoms pose a similar problem
because they are extremely
small. Even with a microscope,
scientists cannot see the
structure of an atom.
Ancient Greek Models of Atoms
If you cut a piece of aluminum foil in half,
you have two smaller pieces of the same
shiny, flexible substance. You could cut the
pieces again and again. Can you keep
dividing the aluminum into smaller pieces?
Greek philosophers debated a similar
question about 2500 years ago.
Ancient Greek Models of Atoms
The philosopher Democritus believed that
all matter consisted of extremely small
particles that could not be divided. He
called these particles atoms from the Greek
word atomos, which means “uncut” or
“indivisible.”
Ancient Greek Models of Atoms
Aristotle thought that all
substances were made of only
four elements—earth, air, fire, and
water. He did not think there was a
limit to the division of matter.
For many centuries, most people
accepted Aristotle’s views on the
structure of matter. By the 1800s,
scientists had enough experimental
data to support an atomic model.
Dalton’s Atomic Theory
What was Dalton’s theory of the structure of
matter?
Dalton proposed the theory that all matter is
made up of individual particles called atoms,
which cannot be divided.
Dalton’s Atomic Theory
Evidence for Atoms
John Dalton studied the behavior of gases in air.
Based on the way gases exert pressure, Dalton
correctly concluded that a gas consists of
individual particles.
Dalton measured masses of elements that
combine when compounds form. The ratio of
the masses of the elements in each compound
was always the same. In other words,
compounds have a fixed composition.
Dalton’s Atomic Theory
When magnesium burns, it
combines with oxygen. In
magnesium oxide, the ratio of
the mass of magnesium to the
mass of oxygen is always about
3 : 2. Magnesium dioxide has a
fixed composition.
Dalton’s Atomic Theory
Dalton’s Theory
 All elements are composed of atoms.
 All atoms of the same element have the same
mass, and atoms of different elements have
different masses.
 Compounds contain atoms of more than one
element.
 In a particular compound, atoms of different
elements always combine in the same way.
Dalton’s Atomic Theory
Dalton made these wooden spheres as a model to
represent the atoms of different elements. A tiny,
solid sphere with a different mass represents each
type of atom.
Dalton’s Atomic Theory
A theory must explain the data from many
experiments. Because Dalton’s atomic theory
met that goal, the theory became widely
accepted.
Over time, scientists found that not all of
Dalton’s ideas about atoms were completely
correct. They revised the theory to take into
account new discoveries.
Thomson’s Model of the Atom
What contributions did Thomson make to
the development of atomic theory?
Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles.
Thomson’s Model of the Atom
When some materials are rubbed, they gain
the ability to attract or repel other materials.
Such materials are said to have either a
positive or a negative electric charge.
• Objects with like charges repel, or push apart.
• Objects with opposite charges attract, or pull
together.
Thomson’s Model of the Atom
Amber is the hardened form
of a sticky, viscous liquid that
protects trees from insects
and disease. If amber is
rubbed with wool, it becomes
charged and can attract a
feather.
Thomson’s Model of the Atom
Thomson’s Experiments
In his experiments, Joseph John Thomson used
a sealed tube containing a very small amount of
gas.
Sealed tube
Glowing beam
filled with gas at
low pressure
Metal disk
Metal disk
Source of
electric current
Metal disk
Source of
electric current
Thomson’s Model of the Atom
Thomson’s Experiments
In his experiments, Joseph John Thomson used
a sealed tube containing a very small amount of
gas.
Sealed tube
Glowing beam
filled with gas at
low pressure
Positive plate
Metal disk
Metal disk
Source of
electric current
Negative plate
Metal disk
Source of
electric current
Thomson’s Model of the Atom
When the current was turned on, the disks
became charged, and a glowing beam
appeared in the tube.
• Thomson hypothesized that the beam was a
stream of charged particles that interacted with
the air in the tube and caused the air to glow.
• Thomson observed that the beam was repelled by
the negatively charged plate and attracted by the
positively charged plate.
Thomson’s Model of the Atom
Evidence for Subatomic Particles
Thomson concluded that the particles in the
beam had a negative charge because they were
attracted to the positive plate. He hypothesized
that the particles came from inside atoms
because
• no matter what metal Thomson used for the disk,
the particles produced were identical.
• the particles had about 1/2000 the mass of a
hydrogen atom, the lightest atom.
Thomson’s Model of the Atom
Thomson’s Model
Thomson revised Dalton’s model to account for
these subatomic particles.
• The atom has neither a positive nor a negative
charge, but there must always be some positive
charge in the atom.
• The atom is filled with a positively charged mass of
matter that has negative charges evenly scattered
throughout it.
Thomson’s Model of the Atom
Thomson’s model is called the
“plum pudding” model. Today, it
might be called the “chocolate
chip ice cream” model.
The chips represent negatively
charged particles, which are
spread evenly through a mass of
positively charged matter—the
vanilla ice cream.
Rutherford’s Atomic Theory
What contributions did Rutherford make to
the development of atomic theory?
According to Rutherford’s model, all of an
atom’s positive charge is concentrated in its
nucleus.
Rutherford’s Atomic Theory
Rutherford’s Hypothesis
Ernest Rutherford designed an experiment to find out
what happens to alpha particles when they pass
through a thin sheet of gold. Alpha particles are fastmoving, positively charged particles.
• Based on Thomson’s model, Rutherford hypothesized that
the mass and charge at any location in the gold would be
too small to change the path of an alpha particle.
• He predicted that most particles would travel in a straight
path from their source to a screen that lit up when struck.
Rutherford’s Atomic Theory
The Gold Foil Experiment
Deflected
particle
Undeflected
particle
Alpha
particles
Gold atoms
Slit
Beam of alpha
particles
Alpha
particles
Screen
Source of
alpha particles
Nucleus
Rutherford’s Atomic Theory
Discovery of the Nucleus
The alpha particles whose paths were
deflected must have come close to
another charged object. The closer they
came, the greater the deflection.
However, many alpha particles passed
through the gold without being
deflected. These particles did not pass
close to a charged object.
Rutherford’s Atomic Theory
Thomson’s model did not explain all of the
evidence from Rutherford's experiment.
Rutherford proposed a new model.
• The positive charge of an atom is not evenly
spread throughout the atom.
• Positive charge is concentrated in a very small,
central area.
• The nucleus of the atom is a dense, positively
charged mass located in the center of the atom.
Rutherford’s Atomic Theory
The Houston Astrodome
occupies more than nine
acres and seats 60,000
people. If the stadium
were a model for an atom,
a marble could represent
its nucleus.
The total volume of an
atom is about a trillion
(1012) times the volume of
its nucleus.
Assessment Questions
1.
Dalton’s theory did not include which of the
following points?
All elements are composed of atoms.
b. Most of an atom’s mass is in its nucleus.
c. Compounds contain atoms of more than one element.
d. In a specific compound, atoms of different elements
always combine in the same way.
a.
Assessment Questions
1.
Dalton’s theory did not include which of the
following points?
All elements are composed of atoms.
b. Most of an atom’s mass is in its nucleus.
c. Compounds contain atoms of more than one element.
d. In a specific compound, atoms of different elements
always combine in the same way.
a.
ANS: B
Assessment Questions
2. J. J. Thomson’s experiments provided the first
evidence of
atoms.
b. a nucleus.
c. subatomic particles.
d. elements.
a.
Assessment Questions
2. J. J. Thomson’s experiments provided the first
evidence of
atoms.
b. a nucleus.
c. subatomic particles.
d. elements.
a.
ANS: C
Assessment Questions
2. J. J. Thomson’s experiments provided the first
evidence of
atoms.
b. a nucleus.
c. subatomic particles.
d. elements.
a.
Assessment Questions
1.
The concept of an atom as a small particle of matter
that cannot be divided was proposed by the ancient
Greek philosopher, Democritus.
True
False
ANS:
T
4.2 Subatomic Particles
Properties of Subatomic Particles
Protons
Based on experiments with elements other
than gold, Rutherford concluded that the
amount of positive charge varies among
elements.
A proton is a positively charged subatomic
particle that is found in the nucleus of an
atom. Each proton is assigned a charge of 1+.
Each nucleus must contain at least one proton.
Properties of Subatomic Particles
Electrons
The particles that Thomson detected were later
named electrons.
An electron is a negatively charged subatomic
particle that is found in the space outside the
nucleus. Each electron has a charge of 1.
Properties of Subatomic Particles
Neutrons
In 1932, the English physicist James Chadwick
carried out an experiment to show that
neutrons exist. Chadwick concluded that the
particles he produced were neutral because a
charged object did not deflect their paths.
A neutron is a neutral subatomic particle that
is found in the nucleus of an atom. It has a
mass almost exactly equal to that of a proton.
Properties of Subatomic Particles
What are three subatomic particles?
Protons, electrons, and neutrons are
subatomic particles.
Comparing Subatomic Particles
What properties can be used to compare
protons, electrons, and neutrons?
Protons, electrons, and neutrons can be
distinguished by mass, charge, and location
in an atom.
Comparing Subatomic Particles
Everything scientists know about subatomic particles
is based on how the particles behave in experiments.
Scientists still do not have an instrument that can
show the inside of an atom.
Comparing Subatomic Particles
Here are some similarities and differences between
protons, electrons, and neutrons.
• Protons and neutrons have almost the same mass. About
2000 electrons equal the mass of one proton.
• An electron has a charge that is equal in size to, but the
opposite of, the charge of a proton. Neutrons have no
charge.
• Protons and neutrons are found in the nucleus. Electrons
are found in the space outside the nucleus.
Atomic Number and Mass Number
How are atoms of one element different from
atoms of other elements?
Atoms of different elements have different
numbers of protons.
Atomic Number and Mass Number
Atomic Number
 The atomic number of an element is the
number of protons in an atom of that
element.
 All atoms of any given element have the same
atomic number. Each hydrogen atom has one
proton in its nucleus. Hydrogen is assigned
the atomic number 1.
 Each element has a unique atomic number.
Atomic Number and Mass Number
Each element has a different atomic number. A
The atomic number of sulfur (S) is 16.
B The atomic number of iron (Fe) is 26.
C The atomic number of silver (Ag) is 47.
Atomic Number and Mass Number
Atoms are neutral, so each positive charge in
an atom is balanced by a negative charge. That
means the atomic number of an element also
equals the number of electrons in an atom of
that element.
• Hydrogen has an atomic number of 1, so a
hydrogen atom has 1 electron.
• Sulfur has an atomic number of 16, so a sulfur
atom has 16 electrons.
Atomic Number and Mass Number
Mass Number
The mass number of an atom is the sum of
the protons and neutrons in the nucleus of
that atom. To find the number of neutrons in
an atom, you need the mass number of the
atom and its atomic number.
The atomic number of aluminum is 13. An
atom of aluminum that has a mass number of
27 has 13 protons and 14 neutrons
Isotopes
What is the difference between two isotopes
of the same element?
Isotopes of an element have the same
atomic number but different mass numbers
because they have different numbers of
neutrons.
Isotopes
Isotopes are atoms of the same element that
have different numbers of neutrons and
different mass numbers.
To distinguish one isotope from another, the
isotopes are referred by their mass numbers.
For example, oxygen has 3 isotopes: oxygen-16,
oxygen-17, and oxygen-18.
All three oxygen isotopes can react with
hydrogen to form water or combine with iron
to form rust.
Isotopes
With most elements, it is hard to notice any
differences in the physical or chemical
properties of their isotopes. Hydrogen is an
exception.
Hydrogen-1 has no neutrons. (Almost all
hydrogen is hydrogen-1.) Hydrogen-2 has one
neutron, and hydrogen-3 has two neutrons.
Because a hydrogen-1 atom has only one proton,
adding a neutron doubles its mass.
Isotopes
Water that contains hydrogen-2 atoms in place of
hydrogen-1 atoms is called heavy water. Hydrogen-2
atoms have twice the mass of hydrogen-1 atoms, so the
properties of heavy water are different from the
properties of ordinary water.
Assessment Questions
1.
In which way do isotopes of an element differ?
number of electrons in the atom
b. number of protons in the atom
c. number of neutrons in the atom
d. net charge of the atom
a.
Assessment Questions
1.
In which way do isotopes of an element differ?
number of electrons in the atom
b. number of protons in the atom
c. number of neutrons in the atom
d. net charge of the atom
a.
ANS: C
Assessment Questions
1.
Of the three subatomic particles that form the
atom, the one with the smallest mass is the neutron.
True
False
Assessment Questions
1.
Of the three subatomic particles that form the
atom, the one with the smallest mass is the neutron.
True
False
ANS:
F, electron
4.3 Compounds
Have you ever wondered what
produces the different colors in a
fireworks display?
Certain compounds will produce
certain colors of light when they
are heated. Compounds
containing the element strontium
produce red light. Compounds
containing barium produce green
light.
Bohr’s Model of the Atom
What can happen to electrons when atoms
gain or lose energy?
Bohr’s Model of the Atom
As did Rutherford's atomic model, Bohr’s atomic
model had a nucleus surrounded by a large volume
of space. But Bohr’s model focused on the electrons
and their arrangement.
In Bohr’s model, electrons move with constant
speed in fixed orbits around the nucleus, like
planets around a sun. Each electron in an atom has
a specific amount of energy.
Bohr’s Model of the Atom
Energy Levels
When an atom gains or loses energy, the
energy of an electron can change.
• The possible energies that electrons in an atom
can have are called energy levels.
• An electron cannot exist between energy levels.
Bohr’s Model of the Atom
An electron in an atom can move from one
energy level to another when the atom gains
or loses energy.
Electron
Electrons gain or
lose energy
when they move
between fixed
energy levels
Nucleus
Bohr Model
Bohr’s Model of the Atom
An analogy for energy levels of electrons is a
staircase.
• The landing at the bottom of the staircase is the lowest
level.
• Each step up represents a higher energy level.
• The step height represents an energy difference
between levels.
• You can only move in whole numbers of stairs.
Bohr’s Model of the Atom
An electron may move up or down two or more
energy levels if it gains or loses the right amount of
energy.
The size of the jump between energy levels
determines the amount of energy gained or lost.
No two elements have the same set of energy levels.
Bohr’s Model of the Atom
Evidence for Energy Levels
Scientists can measure the energy gained when
electrons absorb energy and move to a higher
energy level or measure the energy released
when the electron moves to a lower energy
level. Light is a form of energy that can be
observed.
Bohr’s Model of the Atom
The movement of electrons between energy levels
explains the light you see when fireworks explode.
• Heat causes some electrons to move to higher energy
levels.
• When those electrons move back to lower energy
levels, they release energy. Some of that energy is
released as visible light.
• Different elements emit different colors of light
because no two elements have the same set of energy
levels.
Electron Cloud Model
What model do scientists use to describe how
electrons behave in atoms?
An electron cloud is a visual model of the most
likely locations for electrons in an atom.
Scientists use the electron cloud model to
describe the possible locations of electrons
around the nucleus.
Electron Cloud Model
Bohr’s model was improved as scientists made
further discoveries. Bohr correctly assigned energy
levels to electrons, but electrons do not move like
planets in a solar system.
Today, scientists use probability when trying to
predict the locations and motions of electrons in
atoms. An electron cloud is a visual model of the
most likely locations for electrons in an atom.
Electron Cloud Model
The electron cloud model replaced Bohr's vision of
electrons moving in predictable paths.
The electron cloud is a
visual model of the
probable locations of
electrons in an atom.
The probability of
finding an electron is
higher in the denser
regions of the cloud.
The nucleus
contains
protons and
neutrons
Electron Cloud Model
Electron Cloud Model
The photographs below provide an analogy for an
electron cloud. When the propeller of an airplane is
at rest, you can see the location of the blades.
When the propeller is moving, you see only a blur
that is similar to a drawing of an electron cloud.
Electron Cloud Model
What model do scientists use to describe how
electrons behave in atoms?
An orbital is a region of space around the
nucleus where an electron is likely to be found.
The electron cloud represents all the orbitals in
an atom.
An electron cloud is a good approximation of
how electrons behave in their orbitals.
Electron Cloud Model
For an analogy to the concept of an orbital, imagine
a map of your school. Mark your exact location with
a dot once every 10 minutes over a period of one
week. The dots on your map are a model of your
“orbital.” They describe your most likely locations.
• The places you visit the most would have the highest
concentration of dots.
• The places you visit the least would have the lowest
concentration of dots.
Electron Cloud Model
The level in which an electron has the least
energy—the lowest energy level—has only one
orbital. Higher energy levels have more than
one orbital.
Electron Configurations
What is the most stable configuration of
electrons in an atom?
Electron Configurations
An electron configuration is the arrangement of
electrons in the orbitals of an atom.
When all the electrons in an atom have the lowest
possible energies, the atom is said to be in its
ground state.
Electron Configurations
The most stable electron configuration is the
one in which the electrons are in orbitals with
the lowest possible energies.
Electron Configurations
A lithium atom has three electrons.
• In the ground state, two of the lithium electrons are in the
•
•
•
•
orbital of the first energy level.
The third electron is in an orbital of the second energy
level.
If a lithium atom absorbs enough energy, one of its
electrons can move to an orbital with a higher energy.
This configuration is referred to as an excited state. An
excited state is less stable than the ground state.
Eventually, the electron that was promoted to a higher
energy level loses energy, and the atom returns to the
ground state.
Electron Configurations
The ground state of a person is
on the floor. A gymnast on a
balance beam is like an atom in
an excited state—not very
stable.
When she dismounts, the
gymnast will return to a lower,
more stable energy level.
Assessment Questions
1.
According to Bohr’s model of the atom, which of the
following can happen when an atom gains energy?
An atom returns to its ground state.
b. A neutron can be changed into a proton.
c. A proton can move to a higher energy level.
d. An electron can move to a higher energy level.
a.
Assessment Questions
1.
According to Bohr’s model of the atom, which of the
following can happen when an atom gains energy?
An atom returns to its ground state.
b. A neutron can be changed into a proton.
c. A proton can move to a higher energy level.
d. An electron can move to a higher energy level.
a.
ANS: D
Assessment Questions
2. How does the modern atomic theory describe the
location of electrons in an atom?
Electrons move randomly in space around the nucleus.
b. Electrons can be described as a cloud based on probable
locations.
c. Electrons orbit the nucleus in the same way that planets
orbit the sun.
d. Electrons move in a spiral pattern if increasing distance
from the nucleus.
a.
Assessment Questions
2. How does the modern atomic theory describe the
location of electrons in an atom?
Electrons move randomly in space around the nucleus.
b. Electrons can be described as a cloud based on probable
locations.
c. Electrons orbit the nucleus in the same way that planets
orbit the sun.
d. Electrons move in a spiral pattern if increasing distance
from the nucleus.
a.
ANS: B
Assessment Questions
3. What is meant when an atom is said to be in its
ground state?
There is no net charge on the atom.
b. The number of protons equals the number of neutrons.
c. The atom’s electrons all have the lowest possible energies.
d. It is the isotope with the least number of neutrons.
a.
Assessment Questions
3. What is meant when an atom is said to be in its
ground state?
There is no net charge on the atom.
b. The number of protons equals the number of neutrons.
c. The atom’s electrons all have the lowest possible energies.
d. It is the isotope with the least number of neutrons.
a.
ANS: C