1. 2.3 Periodic Table and Atomic Theory

2. Bohr Diagrams
Diagrams that shows how may electrons are in each shell of an atom
Each shell can only hold a maximum number of electron
Shell 1 = 2
Shell 2 = 8 (total 10)
Shell 3 = 8 (total 18)
**Must fill each shell
completely before filling the next**

4. Bohr Diagram Stable Octet - has a full outer shell
Valence Shell - Outer shell
Valence electrons - electrons in outer shell

5. Drawing a Bohr Diagram:
1. Find out the # of protons, neutrons and electrons
Remember…
Atomic # = # of protons & electrons (in an atom)
Mass # - Atomic # = # of neutrons
2. Start filling electrons in shells starting from the inner most working outward

6. Bohr Diagram
# of protons = atomic # # of protons = 15 # of electrons = # of protons
# of electrons = 15 # of neutrons = mass # - atomic # # of neutrons = # of neutrons = 16
15P
16N

7. Practice Together… Bohr model for: You try… Sodium Chlorine Lithium
Fluorine

8. Noble Gas Stability Noble gases are the most unreactive elements
Their atoms have full valence shells
(they contain the maximum possible number of electrons without going to the next shell)
When elements combine, they are trying to gain a full outer shell of electrons by either giving away their electrons to another element, or taking electrons from another element.
Noble gases are happy the way they are – they do not want to gain or lose electrons. This is why they do not react easily with other elements!

9. Noble Gas Stability
Other elements either gain or lose electrons to try to become like the noble gas nearest to them on the periodic table.
Non-metal atoms will gain electrons.
Metal atoms will lose electrons.

10. How Atoms become Ions
Atoms become ions by gaining or losing electrons.
When this happens, the number of protons and electrons becomes unequal.
Remember that a neutral atom (one with no charge) has an equal number of protons and electrons.

11. Example… Lithium tends to form a +1 ion.
It has 1 valence electron that it loses to become like the closest noble gas on the periodic table, Helium
A neutral lithium atom has 3 protons and 3 electrons
An Li+ ion has 3 protons and 2 electrons
Less electrons = less negatives = +1 charge

12. Example… Oxygen tends to form a -2 ion.
It is missing 2 electrons in it’s valence shell, by gaining two electrons, oxygen becomes like the closest noble gas on the periodic table, Neon
A neutral oxygen atom has 8 protons and 8 electrons.
An O-2 atom has 8 protons and 10 electrons.
It gained 2 electrons = more negatives = -2 charge