A-Level Trends in the elements across a period and down a group: worked solution

Question

Account for the decrease in atomic radius on moving across period 3 from sodium through to chlorine.

Across period 3 from Na to Cl each successive element has one more proton, so the nuclear charge increases. The extra electrons are added to the same (third) principal quantum shell, so the shielding by inner shells stays essentially the same. The effective nuclear charge felt by the outer electrons therefore increases steadily, pulling the outer shell in more strongly, so the atomic radius decreases.