Ionization Enthalpy

Ionization Enthalpy

If Group 13 followed the simple periodic rule, ionisation enthalpy would fall smoothly all the way down the group as atomic size increases. It does not.

• From B → Al, ionisation enthalpy falls, exactly as expected, because the atom is getting bigger.
• Between Al and Ga, and again between In and Tl, there is an unexpected jump upward instead of the expected further decrease.

The cause is again the poorly-shielding d electrons (and, for thallium, f electrons) sitting in the inner core of Ga, In and Tl. Because these electrons fail to screen the nucleus effectively, the increase in nuclear charge down the group is felt more strongly than it "should" be, so ionisation enthalpy rises at these two points instead of continuing to fall.

Two patterns hold true for every member of the group, irrespective of these irregularities:

• The three successive ionisation enthalpies always increase in the expected order: $\Delta_iH_1 < \Delta_iH_2 < \Delta_iH_3$.
• The sum of the first three ionisation enthalpies is very large for every Group 13 element. …