Bohr Model
Starter:
What do you picture when you
think of an atom?
(What does it look like?)
Practice:
Bohr’s Model of the Atom
How to draw the model
Bohr’s Discovery
• Bohr observed the light spectrum of
Hydrogen. If all of the electrons existed
anywhere in the cloud, he would expect to
see a Continuous Spectrum like this:
How Bohr Got This
• When Bohr observed the light spectrum
for Hydrogen, he saw this:
What this meant was
• The atom was more like this:
What it means
• This Line Spectrum shows that electrons
can only exist at certain levels he called
Shells or Orbits.
• The electrons can jump from orbit to orbit,
but can’t exist in between.
• More energy, different color, different line,
different energy level.
• The more energy an electron has, the
higher energy level (orbit) it goes in.
Bohr’s Atomic Theory
The Bohr model was a modification of the
Rutherford model.
Bohr however put the electrons in certain
circular orbits around the nucleus called
shells.
+
Bohr’s Atomic Theory
Protons and neutrons are found in the nucleus.
Electrons orbit the nucleus in various shells.
Everything else is empty space.
1st shell
Nucleus
0
Empty
space
2nd shell
+
3rd shell
Bohr’s Atomic Theory
Each of the shells has a maximum number of
electrons that it can hold.
1st shell
2 electrons
Nucleus
Empty
space
+
2nd shell
8 electrons
3rd shell
18 electrons
Bohr’s Atomic Theory
Ex.1) Draw a Bohr model of Hydrogen-1.
Step-1 Draw a circle to represent the nucleus.
Step-2 Determine the number of protons and
neutrons and place them in the nucleus.
Step-3 Draw a circle around the nucleus to
represent the electron shell.
Step-4 Place the electron in the shell.
1p
0n
H-1
e-
Bohr’s Atomic Theory
Ex.2) Draw a Bohr model of Helium-2.
Step-1 Draw a circle to represent the nucleus.
Step-2 Determine the number of protons and
neutrons and place them in the nucleus.
Step-3 Draw a circle around the nucleus to
represent the electron shell.
Step-4 Place the electron in the shell.
e-
Helium-2
2p
2n
e-
Bohr’s Atomic Theory
Ex.3) Draw a Bohr model of Lithium-3.
Step-1 Draw a circle to represent the nucleus.
Step-2 Determine the number of protons and
neutrons and place them in the nucleus.
Step-3 Draw circles around the nucleus to
represent the electron shells.
Step-4 Place the electrons in the shells.
e-
Litium-3
3p
4n
e- e-
Bohr’s Atomic Theory
Ex.4) Draw a Bohr model of Neon-10.
Step-1 Draw a circle to represent the nucleus.
Step-2 Determine the number of protons and
neutrons and place them in the nucleus.
Step-3 Draw circles around the nucleus to
represent the electron shells.
Step-4 Place the electrons
in the shells.
e- eee-
Neon-10
e-
10p
10n
e- e-
e- e
e-
Application
Using the White boards you build the following atoms:
Sodium
Neon
Boron
Aluminum
Bohr Model of Ions
• What is an Ion?
– An Atom that has lost or gain one or more
electrons, giving it a charge.
– The positive charge tells how many electrons
have been lost.
– The negative charge tells how many electrons
have been gain.
How is charge shown?
• The charge is shown in the upper right hand
corner of the Symbol.
• Examples:
– Na+ - sodium ion has a +1 charge because it lost
one electron.
– O-2 – oxygen ion (known as oxide) has a -2 charge
because it gained two electrons.
• The electrons are lost or gained in the outer level, either
completely empting it, or filling it to eight electrons.
Application
• Using the White boards you build the following
atoms:
• Potassium Ion: K+
• Nitrogen Ion (Nitride): N-3
• Bromine Ion (Bromide): Br• Aluminum Ion: Al3+
Electron Terms
• Ground State: All electrons are in the
lowest energy level. All electrons are in
their proper place.
• Excited State: One or More electrons are
in a higher energy level (orbit) then they
should be.
Where Bohr Fails
• In the 4th Period, electrons start going to a
lower energy level. According to Bohr this
shouldn’t happen.
• In the 4th Period, the first two electrons go
in the 4th Level, but the next ten go in the
3rd Level, then the next six go back in the
4th Level.
• Bohr couldn’t explain this.
Connection
How did “building” your atoms
help you understand Bohr’s
model?
Exit
What are the limitations of Bohr’s
model?