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

Answer is in the pic .
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Answer 2

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Atomic radius of the elements generally decreases from left to the right in a period because on moving from left to right in a period the nuclear charge gradually increases by one unit and one electron is also added in the electron shell. Due to this the electrons get attracted more and more towards the nucleus consequently the atomic radii decrease.

Atomic radius of the elements increases as we move downwords in a group because on moving down a group there is an increase in principal quantum number and thus, increase in the number of electron shells. Therefore the atomic size is expected to increase.