# How should the ionization energies of the alkali metals compare with the ionization energies of the adjacent Group, the alkaline earths?

Aug 7, 2016

A priori , we would expect the ionization energies of an alkali metal to be LESS than that of its alkaline earth neighbour. Why?

#### Explanation:

It is well known that ionization energies INCREASE across a Period from left to right as we observe the Table, but decrease down a Group.

Across the Period, as nuclear charge $Z$ sequentially increases, electrons in the same shell shield each other very imperfectly from the nuclear charge. The result is that atomic radii decrease markedly across the Period.

Down the Group, complete electronic shells shield the nuclear charge effectively. Electrons join an expanded valence shell, and the $\text{aufbau}$ process begins again across the next Period.

Given all this, you are still a physical scientist, and you rely on data, on quantitative measurement. You should look at elemental ionization energies across the Period and down the Group, and see if the rationalization given above is reasonable. Next step is to look at atomic radii, and rationalize this Periodic Trend.