# Hich of the following elements has the smallest first ionization energy? How do you determine this?

## a. sodium b. calcium c. potassium d. magnesium

Oct 17, 2016

Ionization energy decreases DOWN a Group, and increases from left to right across a Period as we face the Periodic Table.

#### Explanation:

The order of ionization energies is thus (smallest to largest):

$\text{potassium"; "sodium"; "calcium"; "magnesium}$.

As a chemist, as a physical scientist, you should, however, always examine the data. Here is a start.

That ionization energy should decrease down a Group, down a column of the Periodic Table, is reasonable, because the valence electron is farther removed from the nuclear core. Across the Period, across a row, from left to right the ionization energy should increase because we add nuclear charge that is imperfectly shielded by the valence electrons.