Question

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

Answer 1

Atomic radius denotes the size of an atom.It is the radius of the atom.
Atomic radius decreases across a period because across period one electron gets added up and nuclear charge increase .So,the nucleus attracts the electrons more and the atom becomes smaller.

Answer 2

atomic radius is the distance between nucleus and outermost shell of the atom of an element.

It decreases in periods because in period, electron, neutron and proton keep on increasing but the shells remain same .