Chemical formulas:

Chemical formulas are short-hand representations of compounds using chemical symbols and oxidation numbers.

The simplest chemical formulas are for binary compounds - compounds made up of two elements.

• Binary compounds have a positive half and a negative half.
   
• The positive half is written first, the negative half second.

Oxidation numbers are used to write chemical formulas.

• The sum of the oxidation numbers in a compound must equal zero.
   
• The oxidation number of many atoms can be read directly from the Periodic Table.
   
• The oxidation number of "transition metals", and any atoms that act in unusual ways, will be provided. The name of the atom will be given with its positive oxidation number as a Roman numeral in parenthesis after the name.

  Example: iron (III) has an oxidation number of +3, while iron (II) has an oxidaiton number of +2

  

• The oxidation number of an ion is equal to its charge.
   
• The sum of the oxidation numbers in a polyatomic ion must equal its charge.