The valency of chlorine with respect to oxygen is ______.

Chlorine shows variable valency and forms several oxides. In chlorine heptoxide, Cl2O7, oxygen has valency 2. Therefore, seven oxygen atoms contribute a total combining capacity of 14, which is shared by two chlorine atoms; hence the valency of each chlorine atom with respect to oxygen is 7. Chlorine belongs to Group 17 of the periodic table and has seven valence electrons. It commonly exhibits oxidation states -1, +1, +3, +5 and +7. In Cl2O7, chlorine is in its highest oxidation state, +7.