Rules for assigning oxidation states

Unit 9 - Redox Reactions

atom in a free/elemental state

it’s oxidation state is 0

i.e. O2, Na, Cl2, HONCl BrIF

group 1 metals

ALWAYS have a +1 oxidation state in their ions and compounds

group 2 elements

ALWAYS have a +2 oxidation state in their ions and compounds

aluminium (group 3)

has an oxidation state of +3 in the majority of its compounds

when hydrogen is bonded to a non-metal

oxidation state of hydrogen is +1

i.e. HCl, HNO3

when hydrogen is bonded to a metal

oxidation state is -1

i.e. NaH, other metal hydrides

oxygen

typically 2, (i.e. H2O and H2SO4)

oxygen exception

in peroxides, oxygen oxidation state is -1

i.e. H2O2, hydrogen peroxide

fluorine

oxidation state is always 1 in all its compounds

i.e. LiF

group 17 halogens

oxidation state is usually -1 in binary compounds (HI, NaCl, KBr) but in combination with oxygen, the oxidation state is positive

neutral molecules

the sum of the oxidation states of ALL the atoms is zero

polyatomic ions

the sum of the oxidation states of all the atoms equals the overall charge of the ion