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How does atomic radius vary in a period and in a group? Give reason for the variation. 

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(a) Atomic radius decreases across a period.

Reason: Within the period as the atomic number increases, electrons are added to the same valence shell and effective nuclear charge increases. Hence atomic size decreases. 

(b)  Atomic radius increases down the group.

Reason: Within the group, number of shells increases. This results in the increase in shielding effect and effective nuclear charge increases. So size of atom increases. 

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