Explain the terms 'valency' and 'variable valency'.

Valency is the combining capacity of an atom or of a radical. The valency of an element or of a radical is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.
Variable valency is the ability of certain elements to have more than one valency or different combining capacities. An atom of an element can sometimes lose more electrons than are present in its valence shell, i.e., there is a loss of electrons from the penultimate shell too. Such element is said to exhibit variable valency.