1. Unit #1 - Chemistry in Action Lesson #4 - Bohr Diagrams
Science 20S
Unit #1 - Chemistry in Action
Lesson #4 - Bohr Diagrams

2. Bohr Diagrams Used to represent the arrangement of electrons in atoms.
Each orbital or shell can accommodate a specific number of electrons.
2,8,8,18,18,32,32
The outermost shell that contains electron is known as the valence shell.
Electrons in this shell are known as valence electrons.

3. Drawing Bohr Diagrams
First draw the nucleus including information about the number of protons and neutrons.
The number of protons is the same as the atomic number found on the periodic table.
The number of neutrons can be determined by taking the atomic mass and subtracting the atomic number.
# of Neutrons = Atomic mass - atomic number

4. Eg. Oxygen

5. Next start drawing the electrons starting at the innermost orbit
The number of electrons is the same as the atomic number
Keep in mind the number of electrons each orbit can accommodate

6. Eg. Oxygen
Inner orbit

7. Valence orbit

8. Try Lithium, Sulfur and Calcium

9. Ions All atoms are trying to have their outer shell full of electrons.
This is known as the Octet Rule.
To get a full outer shell an atom must gain or lose electrons.
An atom that has gained or lost electrons has an electrical charge
It is no longer neutral.
Atoms that have a charge are known as Ions.

10. Positively charged ions are called cations.
Negatively charged ions are called anions.
Determining whether electrons will be gained or lost depends on the number of electrons in the valence shell.
3 or less will lose their electrons.
Metals tend to lose their electrons.
5 or more will gain electrons.
Non-metals tend to gain electrons.
Atoms with 4 electrons in their valence shell can either lose or gain electrons.

11. Bohr Diagrams for Ions First draw the Bohr diagram for the atom
Eg. Lithium Ion

12. Second, decide whether electrons should be added or removed to make a full outer shell and do so.
In this case it is easier to remove 1 electron rather than add 7 electrons

13. Third, draw square brackets, determine the charge and include it in the diagram
Charge = # of protons - # of electrons
3 p+ - 2 e- = 1+

14. Eg. Sulfur Ion
Original Bohr Diagram

15. Decide whether to add or remove electrons
In this case it is easier to add 2 electrons than remove 6 electrons

16. Determine the charge
16 p e- = 2-

17. Try Magnesium, Nitrogen and Aluminum