1. Also called Reduction-Oxidation Reactions
Redox Reactions
Also called Reduction-Oxidation Reactions

2. Redox Reactions
Reactions in which electrons are transferred

3. Mg + Br2  MgBr2
Mg has 2 valence electrons as a neutral atom and then gets a +2 charge in the compound because it lost two electrons to Br.
Br has 7 valence electrons as a neutral atom and then gets a -1 charge (per Br) in the compound because it gained two electrons from Mg

4. Oxidation – the process by which a substance loses one or more electrons
Reduction – the process by which a substance gains one or more electrons

5. O xidation
I s
L oses
R eduction
G ains

6. More terms…
The substance that is oxidized is called the reducing agent
The substance that is reduced is called the oxidizing agent

7. Mg + Br2  MgBr2
Mg is OXIDIZED (because it lost electrons) and is the REDUCING AGENT
Br is REDUCED (because it gained electrons) and is the OXIDIZING AGENT

8. Oxidation Number
The number that represents the bonding electrons

9. Five Rules for Oxidation Numbers
The oxidation number of an neutral atom is ZERO
Ex: Mg has an oxidation number of 0
Br2 has an oxidation number of 0
The oxidation number of any ion is equal to its ionic charge
Ex: Na+1 has an oxidation number of +1
S-2 has an oxidation number of -2

10. In compounds, assign these oxidation numbers to your elements in this order:
Alkali metals (group 1A) are always +1
Alkaline earth metals (group 2A) are always +2
Aluminum is always +3
Fluorine is always -1
Hydrogen will mostly be +1
Oxygen will usually be -2
The algebraic sum of the individual oxidation numbers of all the atoms in a neutral compound must be ZERO
Ex: NaCl (+1) + (-1) = 0

11. The algebraic sum of the individual oxidation numbers of all the atoms in the formula for a polyatomic ion is equal to the charge on the ion.
Ex: CO  all ox # add up to -2

12. Practice Problems KCl H2SO4 KMnO4 CaBr2 Al CO3-2 K+1 Cl-1 H+1 S+6 O-2