Definition oxidation number
Originally, in Chemistry, oxidation was simply regarded as a chemical reaction with oxygen. The reverse process -loss of oxygen - was called reduction. Reaction with hydrogen also came to be regarded as reduction. Later, a more general idea of oxidation and reduction was developed in which oxidation was loss of electrons and reduction was gain of electrons. This definition of oxidation and reduction applies only to reactions in which electron transfer occurs, i.e. to reactions involving ions. It can be extended to reactions between covalent compounds by using concept of oxidation number (or state).
Valency is the measure of an element's ability to combine with other elements, expressed as the number of atoms of hydrogen (or any other standard univalent element) capable of uniting with (or replacing) its atoms. The number of electrons in the outermost shell of the atom dictates the combining ability of an element. The element are described as uni-, di-, tri-, and tetravalent when they unite with one, two, three, and four univalent atoms respectively. Some elements have variable valency: for example, nitrogen and phosphorus have a valency of both three and five. The valency of oxygen is two: hence the formula for water, H2O (hydrogen being univalent).
Oxidation state is the number of electrons to be added (or subtracted) from an atom in a combined state to convert it to elemental form. It is also known as oxidation number.
Oxidation number refers to the Roman numeral often seen in a chemical name, indicating the valency of the element immediately before the number. Examples are lead(II) nitrate, manganese(IV) oxide, and potassium manganate(VII).
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