Why does Kr have larger radius than Ar when Kr has a larger effective nuclear charge?

Question

Looking at the atomic radius trend of the periodic table, Kr has a larger radius than Ar. But my textbook claims that Kr has a larger effective nuclear charge than Ar for n=3. Wouldn't that mean Kr would have a smaller radius than Ar since larger nuclear charge = smaller radius?

Answer 1

The wording of your textbook may be confusing, as Kr doesn't have n=3, it is 4.

The number of electron shells has a greater influence than nuclear charge on atomic size. Krypton has n=4, and Argon has n=3, therefore Krypton has an additional shell that occupies more space. For elements in the same row (same n), larger nuclear charge pulls electrons closer, making atoms smaller. Even though Krypton has larger nuclear charge than Argon, the effect of nuclear charge is smaller than the effect of an additional shell.