The atomic numbers of the elements Na, Mg, K and Ca are 11, 12, 19 and 20 respectively. The element having the largest atomic radius is:
Potassium (K) with an atomic number of 19 has the largest atomic radius. This is because sodium and potassium are elements of group 1. On moving from top to bottom in a group, the atomic radius increases. Magnesium and calcium are elements of group 2. The size of the atomic radius decreases on moving from left to right in a period of the periodic table .