Arrange the following elements in the descending order of atomic size and give a reason for your answer.
Mg, Cl, P, Ar (Atomic numbers of the above elements are 12, 17, 15, 18 respectively).

Explanation

All these elements belong to the third period. This means that their atoms have three shells. Based on the atomic numbers, these elements are arranged in the order : Mg, P, Cl, Ar. Since atomic size decreases along a period, the expected decreasing order is : `Mg (136 pm) gt P (110 pm) gt Cl (99 pm) lt Ar (190 pm).` The element Ar has a large atomic size since it is a noble gas element. It atomic radius is exceptionlly large.