Atomic Structure
Democritus



Believed all matter consisted of extremely small
particles that could not be divided
Called these particles atoms, which means “uncut”
or “indivisible”
He thought there were different types of atoms
with specific sets of properties
◦ Atoms in liquids were round and smooth
◦ Atoms in solids were rough and prickly
Aristotle


Didn’t think there was a limit to the number of
times matter could be divided
Thought all substances were made from only four
elements
◦ Earth
◦ Air
◦ Fire
◦ Water
Dalton’s Atomic Theory: All matter is made up
of individual particles called atoms, which
cannot be divided




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 particular compounds, atoms of different
elements always combine in the same way
J.J.Thomson




Use an electric current to learn more about atoms
Discovered atoms contain negatively charged
particles, or electrons
All atoms are neutral
The atom is filled with a positively charged mass of
matter that has negative charges (electrons) evenly
scattered throughout it.
Ernest Rutherford

Gold Foil Experiment
Deflected particle
Undeflected
particle
Alpha
particles
Gold atoms
Slit
Screen
Alpha
particles
Source of alpha
particles
Nucleus
Ernest Rutherford



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.


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?
a. 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.
Assessment Questions
1.
Dalton’s theory did not include which of the
following points?
a. 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.
ANS: B
Assessment Questions
2.
J. J. Thomson’s experiments provided the first
evidence of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
Assessment Questions
2.
J. J. Thomson’s experiments provided the first
evidence of
a.
b.
c.
d.
atoms.
a nucleus.
subatomic particles.
elements.
ANS: C
Assessment Questions
3.
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
Assessment Questions
3.
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
A proton is a subatomic particle with a charge of
+1 that is found in the nucleus of an atom. Every
nucleus must contain at least one proton.
A neutron is a neutral (has no charge) subatomic
particle that is found in the nucleus of an atom. It
has a the same mass as a proton.
An electron is a subatomic particle with a charge of
-1 that is found outside the nucleus. An electron
has no mass.
The atomic number of an element is the number of
protons in an atom of that element.
Every element has its own atomic number. If the
atomic number of two elements is the same, they
are the same element.
The mass number of an atom is the sum of the
protons and neutrons in the nucleus of that atom.
Every atom of the same element has the same
number of protons and electrons.
Every atom of the same element does not have the
same number of neutrons.
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.


Oxygen has 3 isotopes
◦ oxygen-16
◦ oxygen-17
◦ oxygen-18
Hydrogen has 3 isotopes
◦ hydrogen-1
◦ hydrogen-2
◦ hydrogen-3
Assessment Questions
1.
In which way do isotopes of an element
differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
Assessment Questions
1.
In which way do isotopes of an element
differ?
a.
b.
c.
d.
number of electrons in the atom
number of protons in the atom
number of neutrons in the atom
net charge of the atom
ANS: C
Assessment Questions
2.
Of the three subatomic particles that form the
atom, the one with the smallest mass is the
neutron.
True
False
Assessment Questions
2.
Of the three subatomic particles that form the
atom, the one with the smallest mass is the
neutron.
True
False
ANS:
F, electron
Bohr’s Model of the Atom


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.
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.
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
The movement of electrons between energy
levels explains the light you see when
fireworks explode.



Electrons move to higher energy levels when they
absorb energy
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.
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
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.
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
The electron cloud represents all the orbitals in an
atom.
 An orbital is a region of space around the nucleus
where an electron is likely to be found.


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.
The most stable electron configuration is the one in
which the electrons are in orbitals with the lowest
possible energies.
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?
a.
b.
c.
d.
An atom returns to its ground state.
A neutron can be changed into a proton.
A proton can move to a higher energy level.
An electron can move to a higher energy level.
Assessment Questions
1.
According to Bohr’s model of the atom, which
of the following can happen when an atom
gains energy?
a.
b.
c.
d.
An atom returns to its ground state.
A neutron can be changed into a proton.
A proton can move to a higher energy level.
An electron can move to a higher energy level.
ANS: D
Assessment Questions
2.
How does the modern atomic theory describe
the location of electrons in an atom?
a. 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.
Assessment Questions
2.
How does the modern atomic theory describe
the location of electrons in an atom?
a. 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.
ANS: B
Assessment Questions
3.
What is meant when an atom is said to be in
its ground state?
a. 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.
Assessment Questions
3.
What is meant when an atom is said to be in
its ground state?
a. 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.
ANS: C