Question

What is the first ionization energy?

Answer 1

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(g) + Delta_1 rarr M^(+)(g) + e^(-)`

And thus `Delta_1-="first ionization energy.."`

And likewise...

`M^(+)(g) + Delta_2 rarr M^(2+)(g) + e^(-)`

And thus `Delta_2-="second ionization energy.."`

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