Question

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

Answer 1

In increasing order of atomic radius: `P`; `Al`; and `Ca`.

Explanation:

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.

As a chemist, as a physical scientist, you should seek the data that informs this argument. In other words, you should get a table of atomic size in usable units, and see how it correlates to its position on the Periodic Table.