In a period atomic size --------- moving left to right in a period.

Question

In a group, atomic size -------- moving down the group.

Answer 1

decreases

On moving from left to right in a period, the atomic size decreases as the atomic number (i.e., the number of protons and electrons) increases but the number of shells remain the same. 

Therefore, the force of attraction between the nucleus and the outermost shell of electrons (effective nuclear charge) increases, thus leading to a decrease in size.