What is Oxidation State

Oxidation States

• The number allotted to an element in a compound representing the number of electrons lost or gained by an atom of the element of the compound is called oxidation state.

For example, the electron configuration of copper is [Ar] 3d¹⁰ 4s¹. It attains noble gas configuration by losing one electron. The energy required to lose one more electron is very less and hence copper loses 2 electrons and forms Cu²⁺ ion. Therefore copper exhibits +1 and +2 oxidation state. But +2 oxidation states are more common.

It forms compounds like CuCl₂ and also with oxygen like CuO. In both the cases the oxidation state of Cu is +2.

• Transition elements exhibit varying oxidation states due to the minor energy difference between ns and (n -1) d orbitals.
• Along with ns electrons, (n -1) d electrons takes part in bonding. But due to the availability of few electrons for bonding Scandium does not show variable oxidation states.
• Due to presence of more d electrons, zinc has less orbital available for bonding and hence does not exhibit varying oxidation state.
• Among d-block elements the elements belonging to 8^th group exhibit maximum oxidation state.
• Among the elements of 3d –series Manganese belonging to 7^th group exhibits maximum oxidation state.
• Among the elements of 4d-Series Ruthenium belonging to 8th group exhibits maximum oxidation state.
• Among the elements of 5d-Series Osmium belonging to 8th group exhibits maximum oxidation state.
• The oxidation number of a free element is always 0.
• Oxidation number of (group I) elements like Li, Na, K, Rb, Cs is +1.
• Oxidation number of (group II) elements like Be, Mg, Ca, Sr, Ba is +2.
• Oxidation number of oxygen is -2.
• For example, oxidation state of Phosphorous in the compound HPO₃^2- can be calculated by the following method:

Oxidation state of H = +1

Oxidation state of O = -2

Oxidation state of O₃ = 3(-2)  [Since it has 3 atoms of oxygen.]

Overall oxidation state of the compound = -2

Let P represent the oxidation state of Phosphorous.

Therefore,

HPO₃^2- = +1+P+3(-2) = -2

• P = +3