1. Chapter 4
The Bohr Model

2. The Bohr Model
Electrons move in ORBITS around the positively charged nucleus in an atom.
The nucleus contains +1 protons and neutral neutrons

3. The Bohr Model Electrons move in orbits of fixed size.
These orbits limit how close or far away an electron can get from the nucleus.

4. The Bohr Model
Orbits are “quantized” – only orbits of specific energy exist in an atom
An electron must be in an orbit & can have only the corresponding energy
e’s further out have higher
energy
e’s closer to nucleus have lower
Principal quantum number
n = 1, 2, 3, 4………..
(We’ll study in CH 5)

5. Ground State Ground state is the most stable state for an atom
The electrons are travelling in the lowest available energy orbit
or “shell”

6. Excited State
Light is emitted by the electron as it falls from the excited state back to ground state.

7. Bohr Model
Bohr’s Model explains why hydrogen emits very, very specific kinds of light.
Excited Hydrogen
Gas

8. Bohr Model
Electrons falling from specific levels cause the different wavelengths of light
Excited Hydrogen
Gas

9. Bohr Model – The Rules Every Bohr model atom is drawn the same way.
The nucleus is in the center
2 p+
2 n0

10. Bohr Model – The Rules e- e-
There are electron orbits surrounding the nucleus
In those shells are electrons e-
2 p+
2 n0 e-

11. Bohr Model – The Rules e- e-
Each shell can hold a specific number of electrons
The outer shell can be partially empty, but only if the inner shells are already full e-
2 p+
2 n0 e-

12. 2 e- 8 e- 18 e- 32 e- 50 e- Bohr Model – The Rules
First Shell: ____________________
Second Shell: _______________________
Third Shell: _______________________
Fourth Shell: _____________________
Fifth Shell: ______________________
8 e-
18 e-
32 e-
50 e-

13. Bohr Model – Example Diagram the bohr model of carbon-14 This C has:+ - - + + - - + + + -

14. Bohr Model – Example Diagram the bohr model of magnesium-22
This Mg has:
_______ p+
_______ n0
_______e - - - - - + + - + - + + - + - - + + + + + - + - -

15. Bohr Model – Your Turn!
Diagram the bohr model of the following isotopes
Oxygen – 17
Argon – 35
Titanium – 40
Iron – 57

16. Bohr Model Poster On a piece of construction or computer paper:
Create a Bohr Model for an isotope
An isotope of elements: 3-38
Must get instructor to approve isotope before starting
Use your notes to draw it correctly
Must use 4 different colors in your model
Bonus for artistic ability

17. Chapter 4
Average Atomic Mass

18. Atomic Mass In case you forgot: Weighted Average
Based on ALL isotopes of an element
This is why it is not a whole number
NOT the same thing as mass number

19. Atomic Mass Unit of Atomic Mass = AMU
1 AMU = 1/12th the mass of Carbon-12

20. Atomic Mass Hydrogen has two stable isotopes:
The average atomic mass of hydrogen:
amu

21. Atomic Mass Which isotope is more abundant?
Is there more H-1? More H-2? How can you tell?
Because the average mass ( amu) is closer to the mass number of H-1:
There is more hydrogen-1 in the universe!

22. Steps for Average Atomic Mass
Convert the percent abundance to decimal
This is known as relative abundance
Do this by dividing by 100
Multiply the abundance of each by their mass number.
Add the new numbers together.
This is your average atomic mass!

23. Example:
The element gallium has two stable isotopes: Ga-69 and Ga-71. Ga-69 has a percent abundance of 60.11% and Ga-71 has a percent abundance of 39.89%. What is the average atomic mass of Ga?
1) Convert % to decimal.
60.11/100
39.89/100
2) Multiply decimal by mass #
* 69
* 71 = = = = +
3) Add the products together
69.80 amu

24. Example:
Thallium has two stable isotopes, Th-203 and Th-205. Th-203 has an abundance of 29.52%. Th-205 has an abundance of 70.48%. What is the average atomic mass of Thallium?
1) Convert % to decimal.
29.52/100
70.48/100
2) Multiply decimal by mass #
* 203
* 205 = = = = +
3) Add the products together
amu

25. Your Turn! Do the problems on your practice worksheet
Your instructor will circulate to make sure you are doing it correctly.

26. Calculate % from atomic mass.
Steps for Calculating Percent Abundance (When given mass, but not % abundance)
Because percent abundances will always add up to 100% assign one isotope to have a percentage of X and the other isotope to have a percentage of 1-X

27. Calculate % from atomic mass.
Look up the average atomic mass of the atom on the periodic table.
Set up your problem so it looks like the setup below:
X*(Mass:Isotope-A) + (1-X)*(Mass:Isotope-B) = Average Atomic Mass

28. Calculate % from atomic mass.
Solve for “X.
Multiply by 100 to turn it into a percentage. This is the percent abundance of isotope A.
The percent abundance of isotope B is
100% - % Abundance Isotope-A

29. Example
Hydrogen comes in two stable isotopes, H-1 and H-2. The mass of H-1 is 1.00 AMU. The mass of H-2 is 2.00 AMU. Determine the percent abundance of each isotope.
1) Set abundance = X
% H-1 = X
% H-2 = 1-X
2) Look up AAM
amu
3) Plug into equation
X (1.00) + (1-X)(2.00) =
4) Solve for X
1X +2-2x=
5) Multiply X by 100 to get percent.
* 100
=99.21% H-1
6) Subtract % of A to get % of B
100 – = 0.79% H-2
-1X =
X =

30. Now you Try!
IN YOUR NOTEBOOK ANSWER THIS: Oxygen comes in two stable isotopes, O-16 and O-18. The mass of O-16 is AMU. The mass of O-18 is AMU. Determine the percent abundance of each.
Set abundance
= X
% O-16 = X
% O-18 = 1-X
2) Look up AAM
amu
3) Plug into equation
X (15.99)+(1-X)(17.99) =
4) Solve for X
15.99X x=
5) Multiply X by 100 to get percent.
* 100
=99.53% O-16
6) Subtract from 100
100 – = 0.47% O-18
-2X =
X =