Question

The oxidation number of hydrogen is $ + 1$.

Answer 1

Hint: Hydrogen is the first element in the modern periodic table.It has an atomic number 1.It bonds with various elements. It is present in the form of a diatomic gas ( ${H_2}$ ) which is odourless,tasteless and colourless.Apart from that it is highly flammable.

Complete answer: Atoms of elements always seek to attain stability.This stability is achieved by completely filling the valence electron shell.In order to fill the valence electron shell atoms will accept,donate or share electrons.This give and take of electrons is what gives rise to molecules.Whether an element will accept,donate or share electrons depends on factors like electronegativity, electronegativity of other elements with whom bond formation takes place etc.This give or take of electrons in molecules is indicated by a parameter known as oxidation number.Atoms with positive oxidation number donate electrons and the elements with negative oxidation number accept electrons.
Let us a consider a water molecule which is represented by ${H_2}O$ .In a water molecule two hydrogen atoms are bonded to one oxygen molecules.Since the water molecule as whole has no charge,it’s oxidation number is zero.Therefore the oxidation number of Hydrogen is $ + 1$ and that of oxygen molecule is $ - 2$ .It’s important to note that the oxidation number of hydrogen is always $ + 1$ when it’s in bond formation with non-metals
However when hydrogen forms bonds with metals which are more electropositive than itself,it’s oxidation number changes to $ - 1$ .For example in hydrides like $NaH$ .Here the oxidation number of hydrogen is $ - 1$ since sodium is more electropositive than hydrogen.

Note:
When hydrogen bonds with oxides and regular compounds it’s oxidation number is $ + 1$ and when it bonds with metals which are more electropositive than itself it’s oxidation number is $ - 1$ .In other words the chemical nature of the atoms with which hydrogen bonds greatly affect its state of oxidation.