In each of the following pairs, choose the atom having the bigger size:
(a) Mg (At. No. 12) or Cl( At. No. 17)
(b) Na (At. N0. 11) or K (At. No. 19)
(a) Mg is bigger in size because as we move left to right in period the atomic size decreases.
(b) K is bigger in size because as we move down the group the atomic size increases.