1 It is an imaginary or apparent charge developed over atom of an element when it goes from its elemental free
state to combined state in molecules.
2 It is calculated on basis of an arbitrary set of rules.
The following rules are helpful in calculating oxidation number of the elements in their different compounds. It is to
be remembered that the basis of these rule is the electronegativity of the element
The acids which contain hydrogen , oxygen and a metal or non-metal.
In this case , more than one possibilities arise because of the presence of different number of oxygen atoms. An example
of such an oxoacid series is as follows: HClO, HClO2, HClO3, HCIO4. All these contains same three elements but differ in
the number of oxygen atoms present.
Rule : Names of positive ions end in-ium if the ion has only one oxidation
Rule : If the cation has variable valency (charge), charge is specified in roman numerals in round brackets immediately
after the name of metal atom. For example , Sn2+ is written as tin (II) ion.
Rule : Names of negative ions from hydracids end in –ide.
For example , Cl– (chloride) from HCl, and CN– (cyanide) from HCN. Following examples will
give you a better insight in this nomenclature. It is also useful to remember them.
Anion derived from an oxyacid by removal of one or more H+ ions is termed as oxyanion.
Rule : If the oxyacid is – ic acid, suffix - ate is used with oxy-anion.
Polyprotic acid. Any acid containing more than one replacable hydrogens is said to be a polyprotic acid.
1 These symbols are based on first letter of numbers from 0 to 9. The names of these number are derived
from Greek and Latin languages.
2 The Latin words for various digits of the atomic number are written together in the order of which makes the
atomic number and suffix ‘ium’ is added at the end. In case of bi and tri one ‘i’ is omitted.