1 Ionization energy (ionization potential)

• If an atom gains a moderate amount of energy its electrons are excited to higher levels.
• If the energy is large enough the weakest-bound electron is ejected and the atom becomes a positive ion.

Ionization energy (potential) = the amount of energy required to remove one mole of electrons from one mole of isolated gaseous atoms.

Because one, two or three electrons may be removed there are first, second, third … ionization energies:

• M(g) → M⁺(g) + e⁻ (first ionization energy, IE₁)
• M⁺(g) → M²⁺(g) + e⁻ (second ionization energy, IE₂), and so on.

Unit: kJ/mol. All are endothermic (ΔH positive).

First ionization energies (kJ/mol), selected: H 1312; Li 520, Be 900, B 801, C 1086, N 1402, O 1314, F 1681, Ne 2081; Na 496, Mg 738, Al 578, Si 787, P 1012, S 1000, Cl 1251, Ar 1521; K 419; Cs 376.

Trend in a period:

• generally increases from left to right because the effective nuclear charge increases and the atomic radius decreases, so the nucleus attracts the valence electrons more strongly and more energy is needed to remove one.
• Ionization energy is inversely related to atomic radius.
• (Small irregularities occur at 3A and 6A because of the stability of full and half-filled sublevels, e.g.
• N > O.)

Trend in a group: decreases from top to bottom, because the number of shells increases, the atomic radius increases and the attraction of the nucleus for the valence electrons falls, so less energy is required to remove it.

Noble gases have very high first ionization energies because of the stability of their saturated outer level.

Successive ionization energies.

• IE₂ > IE₁ because the electron is removed from a positive ion in which the effective nuclear charge per electron is greater.
• For group 2A elements IE₃ is much bigger than IE₂, because the third electron must be taken from a saturated inner level.
• For magnesium:

• Mg(g) → Mg⁺(g) + e⁻ ΔH = +737 kJ/mol
• Mg⁺(g) → Mg²⁺(g) + e⁻ ΔH = +1450 kJ/mol
• Mg²⁺(g) → Mg³⁺(g) + e⁻ ΔH = +7730 kJ/mol (a sudden large jump: the electron comes from the stable 2p⁶ level)