The Oxidation State or Oxidation number of an atom in a substance is defined as the actual charge of the atom if it exists as a monoatomic ion, or a hypothetical charge assigned to the atom in the substance by simple (or set) rules.
The oxidation number of an atom in an element is always zero. The oxidation number of an atom that exists in a substance as a monoatomic ion equals the charge on that ion.
In ionic compounds, it is usually the number of electrons gained or lost by the atom. For example, in NaCl, the oxidation states of Na and Cl are +1 and -1 respectively. In covalent compounds, it is decided on the basis of the difference in their relative electronegativities. For example, in CC14 ( a covalent compound) in C is +4 and that of Cl is -1.
The oxidation number of a normal element is directly or indirectly related to the group number to which the element belongs in the Periodic Table. The elements of group IA show a +1 oxidation state and those of IIA show a +2 oxidation state.
Oxidation State in P-block Elements
The lighter elements of p-block elements like those of the 2nd and 3rd periods show a variety of oxidation states both positive and negative. Some of the heavier elements of p-block like those of groups IIIA (Ga, In, TI), IVA (Ge, Sn, Pb), VA(As, Sb, Bi), and VIA (Te, Po) show two oxidation states.
These oxidation states differ from each other by two units. The -higher oxidation state for most of the elements is equal to their group number, while the lower oxidation state is equal to (Group No -2). The higher oxidation state is obtained when all the ns and np electrons from the ns2px configuration of p-block elements ( x = 1,2,3 and 4 for the elements of groups IIIA, IVA, VA, and VIA respectively) are lost.
While; the lower oxidation state is obtained when only np-electrons are lost and the ns-electron pair, due to its extra stability, remains inert, i.e. it is not lost. Such a pair of ns electrons are called inert electron pairs and the effect caused by it is known as the inert pair effect.
All the elements of group VA exhibit the -3 oxidation state. N and P also show a +4 oxidation state. All the elements of group VIA exhibit a -2 oxidation state which is the number of vacancies in hell. Similarly, all the elements of group VIIA exhibit a – 1. oxidation state, and Group VIIIA elements usually show a zero oxidation state.
Oxidation states of normal elements
|Elements of 2nd period||Li||Be||B||C||N||0||F|
|Oxidation states Elements of 3rd-period
Elements of 4th-period Oxidation states
Elements of 5th period
It may be seen that on moving from left to right in each period of p-block elements, the positive oxidation states increase while the negative oxidation states decrease. The inertness of ns2 electrons in p-block elements is found only when the ns2-electron is in the 4th or higher principal quantum number. The inert electron pair effect increases as we move down a group.
For example, this effect increases as we move from Ge to Pb through Sn in Group IVA (Ge< Sn <Pb), indicating that this effect is not much marked in Ge compounds.