Oxidation Numbers

Oxidation states are numbers where we can see what has been oxidised, and what has been reduced.
In a compound, each element has a oxidation number. If an element is single, it's number is zero. A positive number shows it has lost electrons and been oxidised, the more positive it is, meaning the more it has been oxidised, whereas a negative shows it has gained electrons and been reduced, the more negative it is being the more it has beeen reduced. Must have +/- sign.

There are some elements that always have the same oxidation states which will need to be remembered. Hence, I'm gonna type em out, becoz I'm silly.
Hydrogen, +1 (Except in metal hydrides where it;s -1)
Group 1's, +1
Group 2's, +2
Aluminium, +3
Oxygen, -2
Fluorine, -1
Chlorine, -1 (unles in a compound with F or O, where it is positive.)

Since all compounds (NOT IONS) are neutral, the sum of oxidation numbers will = zero.
In ions, the sum of the oxidation numbers will equal the charge. In a compound, the one with the most negative number is the most electronegative.

PCl5 = Cl is minus one, and in this, -5, therefore, P must be +5
NH3 = H is always +1, and in this, therefore, three, so N must be -3
HNO3 = O is -2, and therefore, -6, and H is +1, so therefore, to equal zero, N must be +5
H2S = Hydrogen is always +1, therefore, +2, so the sulphur must be -2
SO42- = Four O's = -8, therefore, S must = +6
CuO =O is -2, and therefore Cu is 2+
Cu2O = O is -2, so Cu must be +1