Modern Atomic Theory
Chapter 4.3
Bohr’s Model of the Atom
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.
Bohr’s Model of the Atom
 Bohr believed
 Individual orbitals could only contain so many
electrons
Orbital
Number of Electrons
1
2
2
8
3
8
4
18
 Orbitals are filled from the inside out
Bohr’s Model of the Atom
2 e-’s
8 e-’s
8 e-’s
18 e-’s
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
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
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
Bohr’s Model of the Atom
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
Electron Configuration
Ground state:
All the electrons in an atom have the
lowest possible energies
Most stable electron configuration
Excited State:
Electron(s) have absorbed energy &
are in now in an higher energy orbital
Very unstable  Electron(s) will return
to its original state.
Electron Configuration
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.
Electron Configuration
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.
Bohr’s Model of the Atom
The movement of electrons between energy
levels explains the light you see when fireworks
explode.
• Heat  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.
Electron Cloud Model
Bohr’s model was improved
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)
Electron Cloud Model
Heisenberg’s Uncertainty Principle
Electron Cloud Model
Heisenberg’s Uncertainty Principle
The position and momentum of a
particle cannot be simultaneously
measured with arbitrarily high precision.
Translation:
We cannot pin point the exact location or
speed of an electron at any given point
and time.
We can only approximate based on
statistics
Electron Cloud Model
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.
Electron Cloud Model
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”
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.
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.
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 Configuration Orbitals