1. Atomic Mass = All the Isotopes
“Atomic mass” is the weighted average of the mass numbers of an element’s isotopes
Atomic mass depends in the relative abundance of the isotopes in nature
Atomic mass is shown in the Periodic Table
Isotope
Mass Number
Percent
Weighted Mass Number
Chlorine-35 35
75.78%
26.52
Chlorine-37 37
24.22%
8.96
“Chlorine”
35.48

2. Orbitals = Inside Energy Levels
Within energy levels there are “orbitals”
Orbitals can hold only two electrons
s orbital – 2 e-
p orbitals – 3 x 2 e-
d orbitals – 5 x 2 e-
f orbitals – 7 x 2e-
Different energy levels have different orbitals
1 = s only; 2 = s, p; 3 = s, p, d; 4 = s, p, d, f; 5 = s, p, d, f; 6 = s, p, d; 7 = s, p

3. Orbitals & the Periodic Table
Orbitals fill from low energy to high energy, but there is overlap between energy levels – this is what gives the Periodic Table its unique shape
The Periodic Table tells us which orbital is being filled with the next electron:
Columns 1 & 2 = the s orbital
Columns 13 through 18 = the p orbitals
Columns 3 through 12 = the d orbitals* (*of the energy level one less than the row number)
The Rare Earth Metals = the f orbitals** (**of the energy level two less than the row number)

4. An Expanded Periodic Table

5. Octet Rule = Valence Electrons
Notice that each row of the Periodic Table ends with an element that has 8 electrons in its outermost energy level (the s and p orbitals are filled)
This is a chemically stable configuration (the noble gases!!) that all atoms seek to achieve
This is known as the Octet Rule – the tendency of atoms to give, take, or share electrons so there are 8 electrons in its outermost energy level
Thus it is the electrons in the outermost energy level that dictate the chemical behavior of an element – these electrons are called “valence electrons”

6. Valence Electrons & Columns
For many elements, the number of valence electrons is indicated by its column in the periodic table:
The number of valence electrons for the transition metals and rare earth metals varies – no general rule is possible
Column 1
Column 2
Columns
Column 13
Column 14
Column 15
Column 16
Column 17
Column 18 1 2
(varies) 3 4 5 6 7 8

7. Electron Dot Diagrams
When we are interested in how an atom will behave chemically rather than its entire structure, we can simplify the Bohr Diagram into an Electron Dot Diagram (Lewis Dot Diagram)
We use the element symbol to represent the nucleus and all of the non-valence electrons
And we show the valence electrons as dots – two each right, above, left, and below the symbol as appropriate to the number of electrons

8. Electron Dot Diagrams Elements 1 – 20
Hydrogen 1 H
Electron Dot Diagrams Elements 1 – 20
Helium 2 He
Lithium 3 Li
Beryllium 4 Be
Boron 5 B
Carbon 6 C
Nitrogen 7 N
Oxygen 8 O
Fluorine 9 F
Neon 10 Ne
Sodium 11 Na
Magnesium 12 Mg
Aluminum 13 Al
Silicon 14 Si
Phosphorous 15 P
Sulfur 16 S
Chlorine 17 Cl
Argon 18 Ar
Potassium 19 K
Calcium 20 Ca

9. Electron Dot Diagrams