# Why is M(g) + Delta rarr M^(+)(g) + e^(-) the ionization energy of magnesium?

##### 1 Answer
Oct 1, 2016

Because that's how we specify the ionization energy.

#### Explanation:

Ionization energies are the energy associated with the formation of one mole of gaseous cations, and one mole of gaseous electrons, from one mole of gaseous atoms:

$M \left(g\right) \rightarrow {M}^{+} \left(g\right) + {e}^{-}$; $\Delta E = \text{ionization energy}$

Such a specification allows us to compare electronic structure on the basis of atoms, which allows an insight to atomic structure and energy levels, rather than the structure of a solid material. It is well know that ionization energies INCREASE across a Period (from left to right as we face the Table) and DECREASE down a Group.