(a) Element A is lying in group VII and so, it will have 7 electrons in its outermost shell. A needs one more electron to obtain inert gas configuration and by gaining one electron it will form a monovalent negative ion. Therefore, A is a non-metal.
(b) Since C belongs to period 3 and has three shells while element A is in period 2 and will have only two shells therefore, element C will have large atomic size than element A.
(c) Since element B lies in group 1 and therefore it has 1 valence electron in its shell. It requires one more electron to get stable gas configuration. So its valency is 1.
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