3_1 Group 13 (IIIA) Elements: Boron, Aluminium and Their Trends

Group 13 (IIIA) contains boron, aluminium, gallium, indium and thallium. Every atom has three valence electrons in the configuration

ns^2np^1

, and the usual oxidation state is +3. The group shows one of the sharpest changes in character anywhere in the periodic table. Boron at the top is a hard, black, non-metallic solid, while aluminium just below it is a typical light metal. Most of what makes boron special comes from its small size.

Trends Down the Group

Group 13 elements: selected properties

Property	B	Al	Ga	In	Tl
Configuration	[He]2s²2p¹	[Ne]3s²3p¹	[Ar]3d¹⁰4s²4p¹	[Kr]4d¹⁰5s²5p¹	[Xe]4f¹⁴5d¹⁰6s²6p¹
1st ionization energy (kJ/mol)	801	578	579	558	589
Covalent radius (pm, approx.)	84	121	122	142	145
Melting point (°C)	2076	660	30	157	304
Density (g/cm³)	2.34	2.70	5.91	7.31	11.85

The trends are less tidy than in groups 1 and 2. Atomic size jumps from boron to aluminium, then barely changes from aluminium to gallium. Between those two elements the 3d subshell fills, and d electrons shield the nucleus poorly. Gallium's outer electrons therefore feel a stronger pull than its position suggests, so the atom stays small and its ionization energy is no lower than aluminium's. The same effect, strengthened by the 4f electrons, makes thallium's ionization energy higher than indium's.

Metallic character rises from boron to aluminium and stays metallic below it. Gallium is unusual for its very low melting point: it melts in a warm hand but does not boil until about 2400 °C. Thallium often prefers +1 over +3, because its

6s^2

pair is reluctant to take part in bonding. This inert pair effect becomes stronger down groups 13 and 14.

Where Boron and Aluminium Are Found

Boron is fairly rare. It always occurs combined with oxygen, mostly as borates, the salts of various boric acids. Plants need it in trace amounts for healthy growth. Aluminium is the most abundant metal in the Earth's crust and the third most abundant element after oxygen and silicon. Most of it is locked in aluminosilicate rocks and clays, but it is extracted from bauxite. Gallium, indium and thallium are rare and are recovered as by-products of processing other metal ores.

Important boron and aluminium minerals

Element	Mineral	Formula
Boron	Borax (tincal)	Na₂B₄O₇·10H₂O
Boron	Colemanite	Ca₂B₆O₁₁·5H₂O
Boron	Sassolite (natural boric acid)	H₃BO₃
Aluminium	Bauxite (main ore)	Al₂O₃·xH₂O
Aluminium	Corundum (ruby, sapphire)	Al₂O₃
Aluminium	Cryolite	Na₃AlF₆
Aluminium	Feldspar	KAlSi₃O₈
Aluminium	Kaolin (china clay)	Al₂O₃·2SiO₂·2H₂O

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Why Boron Is the Odd One Out

Boron's atom is tiny and its first three ionization energies add up to a very large total, so it never forms a

\mathrm{B^{3+}}

ion. Instead it shares all three valence electrons in covalent bonds. This one fact explains its unusual behaviour:

• It is the only non-metal in the group, and it is a hard, high-melting solid with a covalent network structure.
• It forms no simple ionic salts such as a sulphate or nitrate, because there is no stable boron cation to pair with the anion.
• Its oxide

  \mathrm{B_2O_3}

  and hydroxide

  \mathrm{B(OH)_3}

  are acidic, while aluminium's are amphoteric and those of the heavier elements become more basic.
• It forms a large family of covalent hydrides, the boranes, such as diborane

  \mathrm{B_2H_6}

  .
• Besides +3, it can show −3 in metal borides such as

  \mathrm{Mg_3B_2}

  .

Electron Deficiency and Addition Compounds

In a molecule such as boron trifluoride,

\mathrm{BF_3}

, boron forms three bonds and has only six electrons in its valence shell, two short of an octet. The compound is electron deficient, which makes it a strong Lewis acid, or electron-pair acceptor. It readily combines with any molecule that has a lone pair to donate, such as ammonia or an ether, to form a molecular addition compound. The new bond is a coordinate bond, and boron becomes tetrahedral with a complete octet.

FB(F)F.N>>F[B-](F)(F)[NH3+]

Aluminium halides are electron deficient too, but aluminium is larger and can accept a neighbour's lone pair without a separate donor. Aluminium chloride vapour exists as

\mathrm{Al_2Cl_6}

dimers held together by bridging chlorine atoms. This Lewis acidity is what makes

\mathrm{AlCl_3}

and

\mathrm{BF_3}

useful catalysts in organic chemistry.

Tips for remembering

• Three electrons, six around boron: BX₃ has only six electrons around boron, so it grabs a lone pair from any donor. That is why boron compounds are Lewis acids.
• Gallium is not bigger than aluminium: ten poorly shielding 3d electrons sit between them, so gallium's atom is about the same size as aluminium's and its ionization energy is no lower.
• Boron never goes B³⁺: losing three electrons from such a small atom costs too much energy, so boron forms covalent bonds instead of ions.