Oxidation Number: Aka oxidation state. Oxidation number sign placed before the number, unlike
ionic.
• Rules in Most Cases:
– Elements- Oxidation number is zero. Even Cl2.
– Most Metals- Oxidation number of group 1, 2 and 3 same as ionic charge, +1 or +2 or +3. Look
at periodic table, to check none of elements are in the metal groups.
• Special Cases:
– Metal-
– H in metal hydride (NaH) is -1.
– Al, a group 3, is always +3.
– F is always -1.
– Non- Metal-
– Oxygen usually -2.
– O in peroxides (H2O2) is -1 as H is +1.
– O bonded to F is +2.
– Cl, Br and Iodine usually -1.
– Cl, Br and I +1 when bonded with F or O.
• Compounds: Sum of oxidation numbers = total charge of compound. Ones above are always that
number, so can work out unknown. Metals take priority over non-metals when assigning.
• The oxidation number represents one atom of an element. The total change will depend on how
many atoms of the element in a compound there are- sub script.
• Roman Numerals: Roman numeral shows the oxidation number of element. As some elements
have more than one ion charges.
• OILRIG: Oxidation is the loss of electrons and increase in oxidation number. Reduction is gain of
electrons and decrease in oxidation number. Both happen in one equation so one element will be
reduced and another will be oxidised. This is called a redox reaction.
• If oxidation number has increased by 2 and has been oxidised, means has lost two electrons.
• What has been oxidised and reduced: Oxygen has been oxidised as oxidation number has
increased from 0 to +2. Fluorine has been reduced as oxidation number has decreased from 0 to
-1. Total changes in oxidation numbers balance so include sub script.
• Half Equations: Iron loses electrons 2Fe -> 2Fe3+ + 6e-
. Chlorine gains electrons 3Cl2 + 6e-
-> 6ClThe electrons gained and lost balance.
• Redox Reactions of Acids: Metal + acid -> salt + hydrogen.
Zn (s) + 2HCl (aq) -> ZnCl2 (aq) + H2 (g).
2Al (s) + 3H2SO4 (aq) -> Al2(SO4)3 (aq) + 6H2 (g).
Named metal dissolves. Bubbles.