Ionisation energy
The first ionisation energy is the energy involved in removing one mole of electrons from one mole of atoms in the gaseous state.
The first ionisation energy of magnesium:
\(Mg (g) \rightarrow Mg^{+} (g) + e^{-}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,744\, kJ\,mol^{-1}\)
The second ionisation energy is the energy required to remove a second mole of electrons:
\(Mg^{+} (g) \rightarrow Mg^{2+} (g) + e^{-}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1460\, kJ\,mol^{-1}\)
The third ionisation energy shows a massive increase because it requires an electron to be removed from magnesium’s second energy level.
\(Mg^{2+} (g) \rightarrow Mg^{3+} (g) + e^{-}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,7750\, kJ\,mol^{-1}\)
Across a period from left to right, the ionisation energy increases.
This is due to the increase in nuclear charge having a greater pull on the electrons and therefore more energy is required to remove electrons.
Going down a group, the ionisation energy decreases.
This is due to the shielding or screen effect of the outer electrons from the nucleus and so the attraction is weaker and they are more easily removed.
A comparison of the first ionisation energies of some alkali metals is shown below.
