Atomic radius across the period.

Atomic radii of elements decreases as we move from left to right acroos a period. as we move from left to right in a period atomic number increases i.e. one electron and proton are added. So nuclear charge also increases (due to proton), but corresponding electron is added in the same principal quantum shell .Thus increased nuclear charge pulls the electron more strongly towards the nucleus thus size of the atom decreases. Therefore atomic raddii across a period decreases.