# What is the first ionization energy?

Aug 13, 2018

Well, this has a specific definition...

#### Explanation:

..namely it is the energy associated with the formation of ONE mole of GASEOUS cations, and one mole of GASEOUS electrons, from ONE MOLE of gaseous atoms...i.e.

$M \left(g\right) + {\Delta}_{1} \rightarrow {M}^{+} \left(g\right) + {e}^{-}$

And thus ${\Delta}_{1} \equiv \text{first ionization energy..}$

And likewise...

${M}^{+} \left(g\right) + {\Delta}_{2} \rightarrow {M}^{2 +} \left(g\right) + {e}^{-}$

And thus ${\Delta}_{2} \equiv \text{second ionization energy..}$

For a discussion as to how ionization energy evolves across the Periodic Table, see this old answer.