Concept 02 / 09The p-Block Elements
Group 13 Elements: Trends, Inert Pair Effect and Anomalous Boron
Electronic configuration and general trends of boron, aluminium, gallium, indium and thallium: atomic radii (Ga smaller than Al), ionisation enthalpy, electronegativity, melting points, the inert pair effect and stability of the +1 state, Lewis acidity of trihalides and back bonding, and reactions with air, acids and alkalis.
Group 13: boron (a non-metal), aluminium, gallium, indium and thallium (metals). Configuration .
Atomic radius (see figure). Increases down the group, but gallium () is smaller than aluminium (). Gallium follows the first transition series; its ten electrons shield the nucleus poorly, so the effective nuclear charge felt by the outer electrons is larger.
Ionisation enthalpy. Decreases from B to Al as expected, but not smoothly afterwards: Ga is about equal to Al, and Tl is higher than In, because poorly shielding and electrons (lanthanoid contraction for Tl) increase the effective nuclear charge. The sum of the first three ionisation enthalpies is very high for B, which is why boron forms covalent compounds and no ion.
Physical properties. Boron is an extremely hard, black, high-melting non-metal (). Gallium has an unusually low melting point () and a very high boiling point (), so it is liquid over a huge range and is used in high-temperature thermometers.
Oxidation states and inert pair effect. Boron and aluminium show only. Down the group, the pair becomes increasingly reluctant to take part in bonding (inert pair effect), because the electrons of Tl penetrate close to the nucleus and are poorly screened by and electrons. So the state becomes more stable: in stability. For thallium, is the more stable state and is a strong oxidising agent.
Trihalides and Lewis acidity. With only six electrons around the central atom, molecules are electron-deficient Lewis acids: . Lewis acidity of boron halides is , the reverse of what electronegativity suggests, because of - back bonding: the filled orbital of F overlaps effectively with the empty of B and partly fills it; the larger , and orbitals of Cl, Br and I overlap poorly. Aluminium chloride completes its octet by dimerising to with chlorine bridges.
Chemical reactions.
Air: boron is unreactive in crystalline form; aluminium forms a protective oxide film. On heating, both form and nitrides (BN, AlN).
Oxide character: acidic, and amphoteric, and basic.
Acids and alkalis: boron does not react with non-oxidising acids. Aluminium dissolves in dilute HCl () and in alkali (); concentrated renders it passive.
Key Takeaways:
- Radius: B < Ga < Al < In < Tl (Ga smaller than Al).
- Inert pair effect: stability of increases down; is more stable than .
- Lewis acidity: because of back bonding.
- acidic; amphoteric; basic.
- Pitfall: the Lewis acidity order of boron halides is opposite to the electronegativity of the halogens.
Brain-Teaser 1: is a stronger oxidising agent than . Explain.
Solution: Because of the inert pair effect, thallium prefers : readily accepts two electrons to become . Aluminium is stable only in and has no lower state to go to, so is not an oxidant.
Brain-Teaser 2: Why does exist as a monomer while dimerises?
Solution: Boron is small, and four large Cl atoms cannot fit around it easily; partial back bonding also reduces its electron deficiency. Aluminium is larger, can accommodate four chlorines, and completes its octet through two Cl bridges in .
Brain-Teaser 3: Why is gallium used in thermometers for measuring high temperatures?
Solution: Gallium melts at (just above room temperature) but boils only at , so it stays liquid over an enormous range. The low melting point comes from its unusual structure of units in the solid.
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