Boron Group Element | Properties & Facts - Britannica

Ionization energies

Every element in the boron group has three electrons in its outermost shell (so-called valence electrons), and for each element there is a sharp jump in the amount of energy required to remove the fourth electron, reflecting the fact that this electron must be removed from an inner shell. Consequently, the elements of the group have maximum oxidation numbers of three, corresponding to loss of the first three electrons, and form ions with three positive charges.

The apparently erratic way in which ionization energies vary among the elements of the group is due to the presence of the filled inner d orbitals in gallium, indium, and thallium, and the f orbital in thallium, which do not shield the outermost electrons from the pull of the nuclear charge as efficiently as do the inner s and p electrons. In Groups 1 and 2 (Ia and IIa), in contrast to the boron group, outer shell (always referred to as n) electrons are shielded in every case by a constant inner set of electrons, in the (n-1)s2(n-1)p6 orbitals, and the ionization energies of these Group-1 and Group-2 elements decrease smoothly down the group. The ionization energies of gallium, indium, and thallium are thus higher than expected from their Group 2 counterparts because their outer electrons, being poorly shielded by the inner d and f electrons, are more strongly bound to the nucleus. This shielding effect also makes the atoms of gallium, indium, and thallium smaller than the atoms of their Group 1 and 2 neighbours by causing the outer electrons to be pulled closer toward the nucleus.

Related Topics: boron aluminum thallium gallium indium (Show more) See all related content Britannica Quiz Facts You Should Know: The Periodic Table Quiz

The M3+ state for gallium, indium, and thallium is energetically less favourable than Al3+ because the high ionization energies of these three elements cannot always be balanced by the crystal energies of possible reaction products. For example, of the simple, anhydrous compounds of thallium in its +3 oxidation state, only the trifluoride, TlF3, is ionic. For the group as a whole, therefore, the M3+ ionic state is the exception rather than the rule. More commonly the elements of the group form covalent bonds and achieve an oxidation state of three by promoting one electron from the s orbital in the outer shell (designated ns orbital) to an np orbital, the shift permitting the formation of hybrid, or combination, orbitals (of the variety designated as sp2). Increasingly down the group there is a tendency toward the formation of M+ ions, and at thallium the +1 oxidation state is the more stable one. The basicity (a property of metals) of the elements also increases in proceeding down the group, as shown by the oxides they form: boric oxide (formula B2O3) is acidic; the next three oxides, of aluminum, gallium, and indium (formulas Al2O3, Ga2O3, and In2O3) are either acidic or basic depending on the environment (a property called amphoterism); and thallic oxide (Tl2O3) is wholly basic.