# How would you arrange the following elements in order of increasing ionization energy: Li, Rb, Cs, Na?

Dec 1, 2016

The element with LARGEST atomic number, $Z$, should have the SMALLEST ionization energy.

#### Explanation:

Ionization energy is the energy associated with the reaction:

$M \left(g\right) + \Delta \rightarrow {M}^{+} \left(g\right) + {e}^{-}$

We examine 4 alkali metals, $L i , Z = 3$; $N a , Z = 11$; $R b , Z = 37$; Cs, Z=55, which come from successive Periods on the Table.

As the valence electron is farther removed from the nuclear core, the energy required for the given reaction should become progressively less.

As a chemist, as a physical scientist, you should consult a Table of ionization energies in order to see whether this rationalization is supported by the data.