Oxidation and reduction is the process of loosing and gaining electrons, and figuring out oxidation states. This section will be on the short side, but is extremely important to chemisty.
Oxidation States
The oxidation state of an atom is the number of electrons that that element will gain or loose when forming bonds. There are three things to keep in mind when figuring out oxidation numbers for elements and compounds.
1. The oxidation state of an unbonded atom is zero, So when an
atom is not bonded to another atom or is bonded to the same atom, it will be zero.
2. Oxidation numbers for all atoms in a molecule must add to
zero.
3. Oxidation numbers for all atoms in a polyatomic ion must add
up to be the charge on the ion.
The following elements have the same oxidation numbers all of the time, unlike most elements which depend on what molecule they are part of.
| Element | Oxidation Number |
| Alkali metals (Li, Na,...) | +1 |
| Alkaline earths (Be, Mg,...) | +2 |
| Group 3A (B, Al,...) | +3 |
| Oxygen | -2 |
| Halogens (F, Cl,...) | -1 |
Oxidation-Reduction Reactions
In a redox (Oxidation-Reduction) reaction, electrons are exchanged by reactants and oxidation states are changed. For instance take the following reaction.
Fe + 2 HCl --> FeCl2 + H2
The oxidation state
of Fe Changes from 0 to +2
The oxidation state of
H Changes from +1 to 0
Because of this we say H was being reduced (Gaining Electrons), and Fe was being Oxidated (Loosing Electrons). A good way to remember this is LEO the lion goes GER. LEO: Loss of Electrons is Oxidation GER: Gain of Elecrtons is Reduction. Now we can write two half reactions, important for balancing the equation:
| Fe --> Fe2+ + 2 e- | Oxidation |
| 2 H+ + 2 e- --> H2 | Reduction |
Now all there is to is to balance, this already is balanced because the number of electrons lost is the same as the number gained. However this will not always happen and you must multiply or divide all of the coefecients for each half-reaction so the electrons are the same.