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Group 13: Chemical Reactivity

The boron family contains the semi-metal boron (B) and metals aluminum (Al), gallium (Ga), indium (In), and thallium (Tl). The boron family adopts oxidation states +3 or +1. The +3 oxidation states are favorable except for the heavier elements, such as Tl, which prefer the +1 oxidation state due to its stability; this is known as the inert pair effect. The elements generally follow periodic trends except for certain Tl deviations:

Boron

Boron tends to forms hydrides, the simplest of which is diborane, \(B_2H_6\). Boron hydrides are used to synthesize organic compounds. One of the main compounds used to form other boron compounds is boric acid, which is a weak acid and is formed in the following two-step reaction:

\[B_2O_{3 \;(s)} + 3 H_2O _{(l)} \rightarrow 2 B(OH)_{3 (aq)}\]

\[B(OH)_{3 \;(aq)} + 2 H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + B(OH)^-_{4\; (aq)}\]

Boron can be crystallized from a solution of hydrogen peroxide and borax to produce sodium perborate, a bleach alternative. The bleaching ability of perborate is due to the two peroxo groups bound to the boron atoms.

Aluminum

Aluminum is an active metal with the electron configuration [Ne] 2s22p1, and usually adopts a +3 oxidation state. This element is the most abundant metal in the Earth's crust (7.5-8.4%). Even though it is very abundant, before 1886 aluminum was considered a semiprecious metal; it was difficult to isolate due to its high melting point. Aluminum is very expensive to produce, because the electrolysis of one mole of aluminum requires three moles of electrons:

\[Al^{3+} + 3e^- \rightarrow Al(l)\]

Aluminum can dissolve in both acids and bases—it is amphoteric. In an aqueous OH- solution it produce Al(OH)4-, and in an aqueous H3O+ solution it produce [Al(H2O)6]3+. Another important feature of aluminum is that it is a good reducing agent due to its +3 oxidation state. It can therefore react with acids to reduce H+(aq) to H2(g). For example:

\[2Al (s) + 6H^+(aq) \rightarrow 2Al^{3+}(aq) + 3H_2(g)\]

Aluminum can also extract oxygen from any metal oxide.  The following reaction, which is known as the thermite reaction, is very exothermic:

\[Fe_2O_3(s) + 2 Al(s) \rightarrow Al_2O_3(s) +2 Fe(l)\]

Gallium

Gallium has the chemical symbol Ga and the atomic number 31.  It has the electron configuration [Ar] 2s22p1 and a +3 oxidation state. Gallium is industrially important because it forms gallium arsenide (GaAs), which converts light directly into electricity. Gallium is also used in conjunction with aluminum to generate hydrogen. In a process similar to the thermite reaction, aluminum extracts oxygen from water and releases hydrogen gas. However, as mentioned above, aluminum forms a protective coat in the presence of water. Combining gallium and aluminum prevents the formation of this protective layer, allowing aluminum to reduce water to hydrogen.[7]

Indium

Indium has the electron configuration [Kr] 2s22p1 and may adopt the +1 or +3 oxidation state; however, the +3 state is more common. Indium is soluble in acids, but does not react with oxygen at room temperature. It is obtained by separation from zinc ores. Indium is mainly used to make alloys, and only a small amount is required to enhance the metal strength.

Thallium

Thallium has the electron configuration [Xe] 2s22p1 and has a +3 or +1 oxidation state. Because thallium is heavy, it has a greater stability in the +1 oxidation state (inert pair effect). Therefore, it is found more commonly in its +1 oxidation state. Thallium is soft and malleable. It is poisonous, but used in high-temperature superconductors.

Diagonal Relationship of Beryllium and Aluminum

Both Be2+ and Al3+ are hydrated to produce [Be(H2O)4]2+ and Al(H2O)63+, respectively. When reacted with water, both compounds produce hydronium ions, making them slightly acidic. Another similarity between aluminum and beryllium is that they are amphoteric, and their hydroxides are very basic. Both metals also react with oxygen to produce oxide coatings capable of protecting other metals from corrosion. Both metals also react with halides that can act as Lewis acids. 

Summary of Boron Group Trends

  1. The chemistry of boron is quite different from that of the heavier Group IIIA (Boron column) elements. It differs from aluminum in the following ways.
    1. Its oxide and hydroxide are acidic, where as those of aluminum are amphoteric.
    2. Boron is a semiconductor which has various polymorphs based on icosohedral boron cages, whereas aluminum is a metal with a close packed structure. Boron is very inert and only attacked by hot concentrated oxidizing acids.
    3. No simple salts of B3+ are known, whereas those of Al3+ are numerous and well documented.
    4. Boron forms a wide range of hydrides, which have cage structures. (AlH3)has a polymeric structure which resembles that of AlF3.
    5. The stereochemistries of many boron compounds are based on trigonal sp2 and tetrahedral sp3 geometries. In the latter the octet rule is obeyed. Aluminum forms many compounds with tetrahedral, trigonal bipyramidal, and octahedral geometries.
    6. Multiple pπ- pπ bonding in boron-nitrogen, boron-oxygen, and boron-fluorine compounds is more significant than for the corresponding aluminum compounds. {BN}x for example adopts a graphite structure.
  2. Aluminum, gallium, indium and thallium all form a range of compounds in the +3 oxidation state and compounds in the +1 oxidation state become progressively more stable down group IIIA.
  3. The oxides of aluminum and gallium are amphoteric and indium and thallium oxides are more basic.
  4. The octahedral aqua-ions [M(OH2)6]3+ are acidic and the pKa values for the equillibria: [M(OH2)6]3+ ó [M(OH2)5(OH)]2+ + H+ are Al, ~5; Ga, 3; In, ~4; Tl, 1; showing the Al3+ aquo-ion is the least acidic and the Tl3+ ion the most acidic.
  5. The MX3 compounds are Lewis acids and Lewis acid strengths decrease in the order: Al > Ga > In.
  6. The stability of the hydrides decreases down the group and there are no stable Tl-H compounds. Extraordinary precautions are required to exclude air and moisture in order to isolate Ga2H6.
  7. Aluminum is resistant to corrosion because of an impermeable oxide layer, but is soluble in non-oxidizing mineral acids. Gallium, indium, and thallium dissolve readily in acids, but thallium dissolves slowly in H2SO4 and HCl.
  8. AlN, GaN, and InN have wurtzite structures analogous to cubic BN, but show no analogue of the graphite-type structure of BN.

References

  1. Petrucci, Ralph H, William Harwood, and F. Herring. General Chemistry: Principles and Modern Applications. 8th Ed. New Jersey: Pearson Education Inc, 2001.
  2. Richard L. Travers, et al. "Boron and Arthritis: The Results of a Double-blind Pilot Study." Journal of Nutritional & Environmental Medicine. 1.2 (1990): 127-132.
  3. Silberberg, Martin. Chemistry: The Molecular Nature of Matter and Change. 4th Ed. New York: McGraw-Hill Science/Engineering/Math, 2004

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