The atomic radius is governed by periodic trends:

1. Across a Period: Atomic radius decreases from left to right due to increasing effective nuclear charge. Mg, P, and Cl belong to the 3rd Period. The order of size is $Mg > P > Cl$. Mg (Group 2): 160 pm P (Group 15): 110 \pm

• Cl (Group 17): 99 \pm

2. Down a Group: Atomic radius increases from top to bottom due to the addition of electron shells. Mg and Ca belong to Group 2. Mg (Period 3): 160 pm Ca (Period 4): Is larger than Mg because it has an extra shell ($n=4$).

Combining these trends, Calcium (Ca) is the largest, followed by Magnesium (Mg), then Phosphorus (P), and Chlorine (Cl) is the smallest. Order: Cl < P < Mg < Ca.