# How would you determine which of these elements has the highest first ionization energy?

Jul 2, 2016

How else but by experiment? But we can nevertheless determine the order.

#### Explanation:

Ionization energies INCREASE across the Period, going from left to right as we face the Table; but DECREASE down a Group.

We have the Noble Gases listed in their Periodic Order. The highest first ionization energy is undoubtedly that of $H e$. Why? Because $H e$ is the smallest atom, and because helium has the greatest effective nuclear charge. Given what I have said, you should be able to determine the order of ionization energies:

${\text{Gas"+Deltararr"Gas}}^{+} + {e}^{-}$