7. Put the following elements in order from smallest to largest atomic radius and explain why:
C.0, Sn, Sr​

O, C,  Sn, Sr

Explanation:

As per the periodic table trends, atomic radius of an atom deceases from left to right and increases down a group this is so because across a period or from left to right, number of protons increases that increases the nuclear charge and nuclear radius decreases. Down the group, atomic radius increases because energy level increases and distance between nucleus and outermost orbital increases.

The elements in order from smallest to largest atomic radius are:

O, C,  Sn, Sr because as per the periodic table trends, oxygen (O) fall into right side of Carbon (C) in the periodic table and Strontium (Sn) and Tin (Sn) fall into down the group where Tin (Sn)  is on the right side of Strontium (Sn).

Hence, the correct order is "O, C,  Sn, Sr".