Which of the following best describes why noble gases have a valency of zero?
Their outermost shell is already complete and stable
Valency is the combining capacity of an atom, decided by how many electrons it must lose, gain or share to complete its outermost shell.
Noble gases already have a complete outer shell – an octet (8 electrons), or a duplet (2 electrons) in helium. They need not lose, gain or share electrons, so their valency is zero.
They are not incapable of forming compounds under every condition – xenon, for example, forms compounds such as XeF₂ with fluorine.
They are not the smallest atoms in their periods; atomic size is unrelated to valency, and radon and xenon are large atoms.
They do have outer-shell electrons – 8 (2 in helium); the shell is full, not empty.
Hence, the answer is Their outermost shell is already complete and stable.
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