Question

Why does atomic size decrease across the period and increase down the group?​

Answer 1

Explanation:

Atomic radius is the distance from the atom's nucleus to the outer edge of the electron cloud. In general, atomic radius decreases across a period and increases down a group. Across a period, effective nuclear charge increases as electron shielding remains constant. ... This results in a larger atomic radius.

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Answer 2

Explanation:

This is a very important periodic phenomenon: the contraction of atomic radii across the period. While as we add to Z (the number of protons in the nucleus), we also add another electron (and charge is therefore kept neutral), the increased nuclear charge acts disproportionately on the valence electrons, and contracts this shell.

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