Atomic size decreases across a period because but the positive charge of the nucleus gets increases across a period.
What is the shielding effect?
The shielding effect is the shielding from the pulling force of the positively-charged nucleus given to. the outermost shell electrons by the inner electrons.
Since the shielding effect remains constant across a group but the positive charge of the nucleus gets bigger, atomic size decreases across a period.
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