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Atomic size decreases across a period.

1. Atomic size is defined as the distance between the centre of the nucleus of an atom to its outermost shell.
2. Across a period, the nuclear charge increases. Nuclear charge is the positive charge on the nucleus of an atom. Also, across the period electrons are added to the same shell. So, electrons experience greater pull from the nucleus.
3. Therefore, atomic size decreases across a period.