4.3 Modern Atomic Theory
What produces the different colors
in a fireworks display?
•Some compounds produce
specific colors of light when they
are heated.
Examples:
•Compounds containing strontium
produce red light.
•Compounds containing barium
produce green light.
4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Bohr’s atomic model (1913):
•Nucleus surrounded by a large volume of space
– (like Rutherford's model of 1911)
•Focused on the arrangement of electrons.
– 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
Bohr’s Model of the Atom
Atom gains or loses energy 
Electron(s) moves between energy levels
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
Analogy: electron energy levels like 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 – you can’t go up
half a step
4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Electron can:
•Move up or down (i.e. gain or lose energy)
•Move one or more energy levels at a time
– if it gains or loses the right amount of energy.
•Size of jump  amount of energy change
(gained or lost)
Each element has unique set of energy levels
4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Light is a form of energy
 It can be observed/measured.
 It can be emitted or absorbed by electrons
 Absorb energy 
move to a higher energy level
 Release energy 
move to a lower energy level
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
each element has its own unique set of energy levels.
4.3 Modern Atomic Theory
Electron Cloud Model
Describes possible locations and energy levels
of electrons around the nucleus.
Electron cloud:
– Visual model
– Most likely locations for electrons
4.3 Modern Atomic Theory
Electron Cloud Model
Electron cloud model replaced Bohr's model
of electrons moving in predictable paths.
The nucleus
contains
protons and
neutrons
Electron Cloud Models
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.
4.3 Modern Atomic Theory
Electron Cloud Model
Bohr’s model was improved
[scientists made further discoveries]
– Electron “orbits” do have specific energy levels.
[Bohr got this part right]
– Electrons do not move like planets in a solar system.
[Bohr got this part wrong]
Electron cloud model:
Visual model based on probability (statistical
equations)
4.3 Modern Atomic Theory
Electron Cloud Model - Analogy:
When propeller stopped, you see location of blades.
When propeller is moving, you see only a blur.
– Blur is similar to drawing of electron cloud.
4.3 Modern Atomic Theory
Electron Cloud Model
Orbital:
– Region of space around the nucleus where
an electron is likely to be found.
Electron cloud:
– All the orbitals in an atom.
Electron cloud = good approximation of how
electrons behave in their orbitals.
4.3 Modern Atomic Theory
Electron Cloud Model - Analogy
Imagine a map of our school.
•Mark your exact location with a dot once every 10
minutes for one week.
•Dots on map are model of your “orbital”
– Shows most likely locations to find you.
• Places visited most  highest concentration of dots.
• Places visited least  lowest concentration of dots.
•Everybody’s dots together = school’s “electron cloud”
4.3 Modern Atomic Theory
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.
4.3 Modern Atomic Theory
Electron Configuration:
Ground state:
– All the electrons in an atom have the lowest
possible energies
– Most stable electron configuration
4.3 Modern Atomic Theory
Lithium atom (Atomic # = 3) has three electrons.
Ground state:
– Two electrons in lowest energy orbital;
– Third electron in next (second) energy level.
Excited state:
•Atom absorbs energy 
Electron moves to higher orbital,
leaving “hole” in lower orbital
•Excited state less stable than
ground state, so
•Electron in higher energy level
loses energy  falls into “hole”
•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
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
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.