(a) (i) Atomic radius decreases from left to right in a period because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the size decreases.
(ii) Atomic radius increases down a group because number of shells increases.
(b) X12- 2, 8, 2
Y16- 2, 8, 6
These two elements belong to 3 period because period of an atom in periodic table is equal to the number of shells present in that atom.
These two elements will form ionic bond because X has two valence electrons so it will completely lose these two electrons to acquire noble gas electronic configuration and Y has six valence electrons and it has more electronegativity so it will gain two electrons to complete its octet. So X will completely give its two electrons to Y and ionic bond will form.
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