Question

what is meant by atomic radius explain why it decreases across a period

Answer 1

As we move from left to right along the period the positive charge from the nucleus increase as the number of protons increases.

Due to that more attraction takes place between nucleus and electrons and they shrink.

So this is the reason to your question.

Hope it helps.

Answer 2

atomic radius. the distance between
centre of atom to the outer most shell.