Valency of an atom is the number of electrons gained, lost or shared so as to complete the octet of electrons in the valence shell.

Valency of chlorine: It has electronic configuration = 2, 8, 7 Thus, one electron is gained to complete its octet and so its valency is 1.

valency of sulphur: it it has electronic configuration = 2, 8, 6. Thus two electrons are gained to complete its octet and hence its valency = 2

Valency of magnesium: it has electronic configuration = 2, 8, 2. Thus, it can lose two electrons to attain octet and hence its valency = 2.