Question

What is the oxidation number of hydrogen in $H_2$ ?

Answer 1

Hint :Oxidation number is the number that succeeds an atom as its subscript. It is a number that denotes the degree of oxidation or reduction of that atom in the compound. A major tip to identify oxidation numbers is to use simple math for calculations. For$H_2$, since it is a free element, its oxidation number is zero.

Complete Step By Step Answer:
The oxidation number, also called the oxidation state of an atom, is the charge that an atom would have if the compound was composed of ions. An atom can have a positive oxidation state or a negative oxidation state. When it has a positive oxidation state, the number represents the number of electrons that have been donated by that atom, whereas when an atom has a negative oxidation state, it has accepted those many electrons. It is very important to identify the simple pattern as this is the crucial part of this concept.

$H_2$ is hydrogen, existing as a diatomic molecule. Here, two atoms of hydrogen are bonded together as such:

$H_2$ is a free element, meaning it is not bound to any atoms other than its own. It does not have an overall charge, and is a neutral molecule. For free elements, the oxidation number is zero. Hence, the oxidation number of hydrogen in $H_2$ is zero as well.
Some examples of free elements having zero oxidation state are: $Na,Fe,O_2,S_8$ et cetera. Their oxidation numbers are zero.

Note :
To calculate oxidation numbers of ions, simple math is used along with following some general rules such that in an ion, all the oxidation numbers must add up to the charge on the ion; group $1$and group $2$ elements will have oxidation states $+1$ and $+2$ respectively; in a neutral compound all oxidation numbers must add up to zero; Hydrogen has $+1$ oxidation state with non-metals and $-1$with metals. These are only a few rules, but over the course of it, practicing to find out oxidation states of ions in simpler molecules in the beginning will help in identifying those in complex molecules.