What is meant by oxidation number of an atom?

Answer 1

The oxidation number of an atom describes the number of electrons that an element has partially/entirely accepted/donated. A neutral atom has oxidation number of 0. When forming an ion, the overall oxidation number is equivalent to its charge.

Answer 2

The oxidation number of an atom is the charge that the atom would have if electrons were transferred completely. It is used to keep track of the electron transfer in chemical reactions. The oxidation number can be positive, negative, or zero depending on the electron transfer.