# Using periodic trends, arrange the following elements in order of increasing atomic radius: Al, Ca, and P and explain how you choose that order?

Mar 10, 2016

In increasing order of atomic radius: $P$; $A l$; and $C a$.
Atomic size decreases across a Period, and increases down a Group. As $Z$ increases, the nuclear charge becomes larger and electronic orbitals contract. Atomic radii contract across a Period from left to right. Incomplete shells (shells whose existence the Periodic Table implies) shield electrons in the same shell very imperfectly, and atomic radii decrease.
Phosphorus ($Z = 15$) is to the right of aluminum ($Z = 13$), on the same Period, and should thus be smaller than the aluminum nucleus. On the other hand, calcium $Z = 20$ is 4th row: the electrons have begun a new shell, and the metal atom should be MUCH larger than than that of aluminum, and phosphorus.