Question

what is atomic size? how does atomic size of elements vary in a period an in a group? give reasons for variations.​

Answer 1

the atomic size is the number of orbits in an element.
in the period the number of electron increase because of that the number of orbits also increase which led in increased size.
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Answer 2

Atomic size is the distance between the nucleus and its outermost shell or half the internuclear distance between their atoms in combined state. It decreases as we move from left to right in a period as the nuclear charge increases across a period but the number of shell/distance between the shell and nucleus increases causing increase in the nuclear force and hence smaller size.