Define valency and variable valency with examples.
Valency:
Variable valency:
Copper (Cu) = Cuprous (Cu²⁺) and cupric (Cu³⁺)
Iron (Fe) = Ferrous (Fe²⁺) and Ferric (Fe³⁺)
Mercury(Hg) = Mercurous (Hg ⁺¹) and mercuric (Hg ⁺²)
Silver (Ag) = Argentous (Ag⁺¹) and argentic (Ag⁺².)
Tin (Sn) = Stannous (Sn⁺²) and Stannic (Sn⁺³)