What is the valency of sulphuric acid?

Valence is the number of bonds an atom can form. Hydrogen (H) always has valence 1, so other elements' valences equal the number of hydrogen atoms they combine with. Thus, oxygen (O) has valence 2, as in water (H2O); nitrogen (N) has valence 3, as in ammonia (NH3); and chlorine (Cl) has valence 1, as in hydrochloric acid (HCl).

The valence depends on the number of unpaired electrons in the outermost (and, in transition-elements, the next) shell of the atom's structure. The sharing of the unpaired (valence) electrons in a bond mimics the stable configuration of the noble-gases, whose outer shells are full. Elements that can achieve stable configurations by various combinations have more than one valence.