4.3 Modern Atomic Theory
4-3 Cornell
• Bohr’s Model of the Atom
– Energy Levels
– Draw an example of Bohr’s Model
– Evidence of energy levels
• What happens when they change energy levels
– Electron Cloud Model
• Draw a picture of it.
• How is it different than Bohr’s Model.
– Atomic Orbitals
– Electron Configurations
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
III. Modern Atomic Theory
A. Bohr’s Model of the Atom
1. Energy levels- The possible energies that electrons
in an atom can have.
a. 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
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
b. An electron may move up or down two or more
energy levels if it gains or loses the right amount
of energy.
c. The size of the jump between energy levels
determines the amount of energy gained or lost.
d. An electron cannot exist between energy levels.
e. No two elements have the same set of energy
levels.
4.3 Modern Atomic Theory
2. Evidence for Energy Levels
a. Scientists can measure the energy gained
when electrons absorb energy and move to a
higher energy level
b. Scientist can measure the energy released
when the electron moves to a lower energy
level.
c. Light is a form of energy that can be
observed.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
B. Electron Cloud- a visual model of the most likely
locations for electrons in an atom
1. Today, scientists use probability when trying to
predict the locations and motions of electrons in
atoms
2. Scientists use the electron cloud model to describe the
possible locations of electrons around the nucleus.
4.3 Modern Atomic Theory
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
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
C. Atomic Orbitals- An orbital is a region of space around
the nucleus where an electron is likely to be found.
1. The electron cloud represents all the orbitals in an
atom
2. An electron cloud is a good approximation of how
electrons behave in their orbitals.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
3. The level in which an electron has the least
energy—the lowest energy level—has only one
orbital.
4. Higher energy levels have more than one orbital.
4.3 Modern Atomic Theory
D. Electron Configurations- the arrangement of electrons in the
orbitals of an atom
1. When all the electrons in an atom have the lowest possible
energies, the atom is said to be in its ground state.
2. The most stable electron configuration is the one in which
the electrons are in orbitals with the lowest possible
energies.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
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