Why Aluminium Pans Don't Rust Even Though Aluminium Is So Reactive

Borax helps solder metals, alum clears muddy water, and aluminium wraps food and carries electricity across the country. All three come from Group 13, a family that begins with a non-metal, boron, and turns increasingly metallic down the group.

This lesson covers the trends in Group 13 and the anomalous behaviour of boron, borax and alums, and the chemistry and uses of aluminium.

**Group 13 elements — boron, aluminium, gallium, indium and thallium — have an $n{s}^{2}n{p}^1$ configuration and usually show the +3 oxidation state, but the +1 state grows more stable down the group, and tiny boron behaves as a non-metal unlike the rest.

General trends:

- Atomic radius increases down the group, but gallium (135 pm) is slightly smaller than aluminium (143 pm), because its poorly shielding 3d electrons let the nucleus pull the outer electrons closer
- Oxidation states: +3 is common, but the inert pair effect** makes +1 more stable for thallium, as the $n{s}^2$ pair resists taking part in bonding
- Trihalides are Lewis acids, because the central atom has only six electrons in its valence shell

Why boron is anomalous:

- Very small size and high ionisation enthalpy, so it forms only covalent compounds
- Its maximum covalency is 4, as in $[\mathrm{B}{\mathrm{F}}_4{]}^{-}$, because it has no d orbitals; aluminium reaches 6, as in $[\mathrm{Al}{\mathrm{F}}_6{]}^{3-}$
- ${\mathrm{B}}_2{\mathrm{O}}_3$ is acidic, while $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ is amphoteric

Worked reasoning — Lewis acid strength of boron trihalides. By electronegativity alone, $\mathrm{B}{\mathrm{F}}_3$ should be the strongest Lewis acid. The actual order is $\mathrm{B}{\mathrm{F}}_3<\mathrm{BC}{\mathrm{l}}_3<\mathrm{BB}{\mathrm{r}}_3$, because the filled 2p orbitals of fluorine donate electron density back into the empty 2p orbital of boron, and this back-bonding works best between orbitals of the same size.

An everyday example. Heat-resistant laboratory beakers and oven dishes are made of borosilicate glass, in which boron oxide helps the glass survive sudden changes of temperature.

The substance. Boron's differences all trace back to size — a tiny atom with no d orbitals cannot form simple ions or expand its covalency.

**Borax is a sodium borate whose anion contains two tetrahedral and two triangular boron units, and alums are double sulphates such as potash alum, ${\mathrm{K}}_2\mathrm{S}{\mathrm{O}}_4\cdot \mathrm{A}{\mathrm{l}}_2(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 24{\mathrm{H}}_2\mathrm{O}$; both are widely used in industry and at home.

Borax, $\mathrm{N}{\mathrm{a}}_2{\mathrm{B}}_4{\mathrm{O}}_7\cdot 10{\mathrm{H}}_2\mathrm{O}$:

- Structure**: better written as $\mathrm{N}{\mathrm{a}}_2[{\mathrm{B}}_4{\mathrm{O}}_5(\mathrm{OH}{)}_4]\cdot 8{\mathrm{H}}_2\mathrm{O}$; its anion has two $\mathrm{B}{\mathrm{O}}_4$ tetrahedra and two $\mathrm{B}{\mathrm{O}}_3$ triangles
- Its solution is alkaline by hydrolysis: $\mathrm{N}{\mathrm{a}}_2{\mathrm{B}}_4{\mathrm{O}}_7+7{\mathrm{H}}_2\mathrm{O}\to 2NaOH+4{\mathrm{H}}_3\mathrm{B}{\mathrm{O}}_3$
- On heating, it loses water, swells and melts into a clear glassy bead: $\mathrm{N}{\mathrm{a}}_2{\mathrm{B}}_4{\mathrm{O}}_7\to 2NaB{\mathrm{O}}_2+{\mathrm{B}}_2{\mathrm{O}}_3$
- Borax bead test: the boron oxide in the bead combines with metal oxides to give coloured metaborates, such as blue copper(II) metaborate, $\mathrm{CuO}+{\mathrm{B}}_2{\mathrm{O}}_3\to \mathrm{Cu}(\mathrm{B}{\mathrm{O}}_2{)}_2$
- Uses: flux for soldering, heat-resistant glass and glazes, the borax bead test, and a mild antiseptic

**Alums, ${\mathrm{M}}_2\mathrm{S}{\mathrm{O}}_4\cdot {\mathrm{M}}_2^{\prime }(\mathrm{S}{\mathrm{O}}_4{)}_3\cdot 24{\mathrm{H}}_2\mathrm{O}$:**

- M is a univalent ion such as ${\mathrm{K}}^{+}$, $\mathrm{N}{\mathrm{a}}^{+}$ or $\mathrm{N}{\mathrm{H}}_4^{+}$; M' is a trivalent ion such as $\mathrm{A}{\mathrm{l}}^{3+}$, $\mathrm{C}{\mathrm{r}}^{3+}$ or $\mathrm{F}{\mathrm{e}}^{3+}$
- Double salts, not complexes: in water they give all their simple ions, such as ${\mathrm{K}}^{+}$, $\mathrm{A}{\mathrm{l}}^{3+}$ and $\mathrm{S}{\mathrm{O}}_4^{2-}$
- Uses: purifying water, stopping bleeding from small cuts, fixing dyes on cloth, and tanning leather

Worked reasoning — why alum clears muddy water. Clay particles in muddy water carry negative charges that keep them apart. The highly charged $\mathrm{A}{\mathrm{l}}^{3+}$ ions neutralise these charges, so the particles clump together and settle.

An everyday example. Phitkari, potash alum, is rubbed on small cuts after a shave at a barber's shop and stirred into pots of muddy water in many villages.

The substance. Alum is a double salt, not a coordination compound — tests on its solution detect potassium, aluminium and sulphate ions separately.

Aluminium is obtained by purifying bauxite to alumina and electrolysing the alumina dissolved in molten cryolite; it is a light, amphoteric metal protected by a thin oxide layer and used from power cables to cooking vessels.

From bauxite to aluminium:

- Bayer process: bauxite is dissolved in hot concentrated sodium hydroxide, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+2NaOH+3{\mathrm{H}}_2\mathrm{O}\to 2Na[\mathrm{Al}(\mathrm{OH}{)}_4]$, leaving impurities such as iron oxide behind
- Pure aluminium hydroxide is precipitated and heated to give pure alumina
- Hall-Heroult electrolysis: alumina dissolved in molten cryolite, $\mathrm{N}{\mathrm{a}}_3\mathrm{Al}{\mathrm{F}}_6$, which lowers the melting point and improves conductivity, is electrolysed; aluminium collects at the cathode

Properties:

- Amphoteric — reacts with both acids and alkalis, releasing hydrogen:

$$2Al+6HCl\to 2AlC{\mathrm{l}}_3+3{\mathrm{H}}_{2}2Al+2NaOH+6{\mathrm{H}}_2\mathrm{O}\to 2Na[\mathrm{Al}(\mathrm{OH}{)}_4]+3{\mathrm{H}}_2$$

- Passive in concentrated nitric acid, which thickens its protective oxide layer
- Strong reducing agent in the thermite reaction: $2Al+\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\to \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3+2Fe$

Uses. Overhead power cables, aircraft and vehicle bodies, food foil, cooking vessels, window frames, and light alloys such as duralumin.

Worked example. In the thermite reaction, 54 g of aluminium (2 mol) reduces 160 g of iron(III) oxide (1 mol) to give 112 g of molten iron (2 mol).

An everyday example. Railway tracks in India are joined by thermite welding, where molten iron from the reaction fills the gap between two rails.

The substance. Aluminium pans do not rust or crumble away even though aluminium is reactive — a thin, tough oxide layer forms at once and seals the metal off from air and water.