Question

reasons for the variable valency

Answer 1

half filled orbitals is the main reason

Answer 2

Reason for Variable Valency -

An atom of an element can sometimes lose more Electrons than are present in its Valence Shell i.e. loss from the Penultimate Shell ( 2nd Last Shell )

If the penultimate shell has not attained stability due to that sometimes one more Electron jumps to the outermost shell thereby increasing the valency Electron ( i.e. Electrons present in the Outermost Shell ). Hence, the atom attains one more electronic configuration and accordingly Valency too.

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