redox

• gain of oxygen • loss of electrons • increase in oxidation number

• loss of oxygen • gain of electrons • decrease in oxidation number

• when a single species is simultaneously both oxidised and reduced • use OS to work this out

• give oxygen to another substance • remove electrons from another substance (so are reduced themselves) • strongest oxidising agents are found in group 7

• remove oxygen from another substance • give electrons to another substance (so are oxidised themselves) • strongest reducing agents are found in group 1

• oxidation/reduction in covalent compounds - instead of true loss/gain of electrons, ‘degree of sharing’ of electrons changes

• OS of an atom in its elemental state = 0 • sum of the OS of all the atoms in a compound = 0 • OS of an ion = the charge on the ion (Cl- = -1) • OS of a group ion = the charge on the ion • OS of an oxygen atom in a compound = −2, except in peroxides it = -1 / when with fluorine it = +2 • OS of a hydrogen atom in a compound = +1, except in metal hydrides it = -1

• If the OS of an element increases, then it has been oxidised. • If the OS of an element decreases, then it has been reduced.

• oxygen = -2 - hydrogen = +1