Question

What is atomic radius? Why does it decrease across a period?

Answer 1

Atomic radius is the distance from Nucleus to the furthermost electron in an atom.

I decreases across a period because there will be more number of electrons and the force of attraction of charge particles increases.

Answer 2

Atomic radius is the radius of the atom.It decreases across a period due the increase in number of electron in last shell.As nucleus has positive charge and electron have negative charge and negative charge and positive charge attract each  other.So the atom shrinks and the atomic radius decreases.