Where is the oxidation number written?

Answer 1

In a molecule the oxidation number is written in ROMAN numerals after the given element.

Although all Group oxidise, any one Group only oxidises by the Group Number, except the Transition Element. These lie between Group '2' & '3' in the Periodic Table.

If we take say, potassium permanganate (KMnO4) The manganese (Mn) has different oxidation states. In the above molecule KMnO4 we could write it as

KMn(VII)O4.

The potassium(K) is a Group '1' element so its oxidation state is Always '1'.

The oxygen (O) has a standard at '-2' . So for four oxygens the overall moiety on oxygen is 4 x -2 = -8

Composing a little sum

Mn + 1 - 8 = 0 (Zero for a neutrally charged molecule).

We have Mn = 8 - 1 = (+)7 (Roman Numerals VII).

The oxidation Number tells us that seven of the manganese electrons in the outer electron energy shell take part in bonding.

Compare to nitrogen in Ammonia (NH3) Nitrogen has five electrons in its outer energy (valance) shell , but only three of those five electrons are involved in bonding to hydrogens, leaving a 'lone pair' of unbonded electrons in nitrogen of ammonia. So nitrogen's oxid'n state in ammonia is '+3'.

Archaically, there is a subtle change of name to indicate different oxidation states.

e.g.

Ferrous chloride (Fe(II)Cl2

Ferric chloride (Fe(III)Cl3

However, remember that Oxidation Number refers to the number of electrons in the outer energy(valance) shell that are involved in Bonding.

Answer 2

The oxidation number is typically written as a superscript to the right of the chemical symbol in a chemical formula. It represents the charge that an atom would have if the compound was ionic.