Ionization Energy and Evidence for Subshells

Ionization energy provides some of the most direct experimental evidence for the way electrons are organized within atoms. The first ionization energy of an element is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in their ground state, forming one mole of gaseous singly charged positive ions. In equation form, $\text{X}(g) \rightarrow \text{X}^+(g) + e^-$. The process is always endothermic because energy must be supplied to overcome the attraction between the negative electron and the positive nucleus. Three main factors determine its magnitude: the nuclear charge (more protons pull electrons more strongly), the distance of the electron from the nucleus (electrons in higher energy levels are held less tightly), and shielding (inner-shell electr