Question

what is variable valence????

Answer 1

an element having more than one valency is called variable valency

Answer 2

in transition elements, an atom loses electrons from the shell next to the valence  shell (penultimate shell). In such a situation the element exhibits more than one valency which is called variable valency. For example: Iron has 2 electrons in its  valence shell. On donating these electrons, its valency is 2+. again...when in excited stage...Fe releases one more electron from its 2nd last subshell...then its valency is 3+