Question

why atomic size decrease left to right and increase up to down in modern peridic table

Answer 1

The atomic size decreases from left to right across the periodic table is because the the valance electrons increases from left to right n thus resulting in shrinking of the distance between nucleons and electrons thus the size decreases.
The atomic size increase from top to bottom down a group because the valance shell increases as when we move top to bottom resulting in increase in atomic size.
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