Question

Define atomic size. how does it vary in a period and in a group ?....Plzz answer .

Answer 1

Hi,
Atomic size is the distance from the nucleus to the valence shell where the valence electrons are located. Atomic size is difficult to measure because it has no definite boundary.The number of energy levels increases as you move down a group as the number of electrons increases. 
Hope it will help you

Answer 2

DEFINITIONS -

atomic size is the distance of the centre of the nucleus from the outermost shell.

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FACTORS ON WHICH IT DEPENDS -

1) number of valence shell -

as the number of valence shell increases atomic size increases .

2) nuclear charge -

as the nuclear charge increases atomic size decreases.

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DOWN A GROUP -

1) number of shells increases .
2) nuclear charge increases.

but the increases in the number of shells dominates over the increase in the nuclear charge.

therefore

atomic size increases
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ACROSS A PERIODS -

1) number of shells remains same.
2) nuclear charge increases.

atomic size decreases