1. Oxidation
Numbers

2. Valence Electrons
Elements are grouped in columns or groups according to their number of valence electrons.
1 2

3. In order to be stable, all atoms want a full outer energy level.
Valence Electrons
In order to be stable, all atoms want a full outer energy level.
Hydrogen and helium’s outer energy level can only hold up to 2 electrons.
Since helium already has 2 valence electrons, its outer energy level is full and Helium is stable. Hydrogen, however, only has 1 valence electron, so hydrogen is unstable.

4. Octet Rule
All the other elements, regardless of how many energy levels they have, want 8 valence electrons in their outer energy level. This is called the octet rule.
All of the noble gases have 8 valence electrons, so they are all stable.

5. Valence Electrons
None of the other representative elements have full outer energy levels, so they are all unstable.

6. Gaining or Losing Electrons
In order to fill their outer energy level, the other atoms can either gain or lose electrons.
Sodium only has 1 valence electron. In order to become stable, sodium will lose its valence electron and drop down to only 2 energy levels.
Sodium Ion
Sodium Atom
Chlorine has 7 valence electrons. In order to become stable, chlorine will gain 1 valence electron to fill its outer energy level. _
Gains outer Cl Cl
Chlorine Atom
Chlorine Ion

7. Gaining or Losing Electrons
As a general rule, atoms with 4 or less valence electrons will lose their valence electrons, while atoms with more than 4 valence electrons will gain electrons.
1 2
Groups 1, 2, 13, and 14 will lose electrons
Groups 15, 16, and 17 will gain electrons

8. Ions
When atoms gain or lose electrons, they develop a charge. Atoms with a charge are called ions.
Lithium atoms have three protons and three electrons, making the overall charge of a lithium atom neutral.
(+3) + (-3) = 0 + - - + + + - -

9. Ions
If lithium loses an electron, then it will have one more positively charged proton than it has negatively charged electrons, so it develops a + 1 charge.
(+3) + (-2) = +1 + - - + + + - -

10. Ions and Oxidation Numbers
All of the atoms in the alkali metals group have 1 valence electron which they lose when they become ions with a +1 charge.
The +1 charge is called the oxidation number.

11. Ions and Oxidation Numbers
All of the atoms in the alkaline earth metals group have 2 valence electrons which they lose when they become ions with a +2 charge.
All alkaline earth metals have an oxidation number of +2.

12. Ions and Oxidation Numbers
All of the atoms in the Boron group have 3 valence electrons which they lose when they become ions with a +3 charge.
All ions in the Boron group have an oxidation number of +3.

13. Ions and Oxidation Numbers
All of the atoms in the Carbon group have 4 valence electrons which they lose when they become ions with a +4 charge.
All ions in the Carbon group have an oxidation number of +4.

14. Ions and Oxidation Numbers
All of the atoms in the Nitrogen group have 5 valence electrons. Each of these atoms will gain 3 electrons when they become ions with a -3 charge
All ions in the Nitrogen group have an oxidation number of -3.

15. Ions and Oxidation Numbers
All of the atoms in the Oxygen group have 6 valence electrons. Each of these atoms will gain 2 electrons when they become ions with a -2 charge
All ions in the Oxygen group have an oxidation number of -2.

16. Ions and Oxidation Numbers
All of the atoms in the Halogen group have 7 valence electrons. Each of these atoms will gain 1 electron when they become ions with a -1 charge
All ions in the Halogen group have an oxidation number of -1.

17. Review Atoms with full outer energy levels are stable
Atoms seek stability
Atoms can lose or gain electrons to fill their outer energy levels
When atoms lose electrons they develop a positive charge
When atoms gain electrons they develop a negative charge
Atoms with a charge are called ions

18. Review
The charge an atom develops when it gains or loses electrons is called its oxidation number
Ions in the same groups have the same oxidation number
Groups on the left hand side of the periodic table tend to lose electrons and have positive oxidation numbers
Groups on the right hand side of the periodic table tend to gain electrons and have negative oxidation numbers

19. Oxidation Numbers -2 +4 +1 -1 +2 -3 +3