How does atomic radius vary in a period and in a group ? How do you explain the variation?

Across the Period : Atomic radii decreases on account of the following facts :
Nuclear charge increases progressively by one unit but the additional electron goes to the same principal shell.
As a result the electron cloud is pulled closer to the nucleus by the increased effective nuclear charge. Across the Group : Increases.
Reason : Nuclear charge increases with increase in atomic number but at the same time, there is progressive increase in principal energy levels.