Question

What is the trend in atomic radius as you go across a period?

Answer 1

Atomic radius decreases across a Period, but increases down a Group.

Explanation:

So why?

Going across the Period, you add protons, positive particles, to the nucleus, whilst electrons are added to the same shell. Nuclear charge wins, and the electrons are pulled closer to the nucleus. Going down a Group, there are inner shells of electrons that shield the increasing nuclear charge. Shielding wins, and the next electrons shells are built on progressively larger shells.