The p-block occupies the right-hand side of the periodic table — Groups 13 to 18. In these elements the last electron enters a p-orbital, giving a general valence configuration of ns2np1-6. Because the p-block spans both metals (Al, Sn, Pb), metalloids (B, Si, Ge) and non-metals (C, N, O), it shows the widest variety of properties of any block. The chemistry of these elements is dominated by the gradual filling of p-orbitals and by the presence of completely filled inner d and f sub-shells in the heavier members.
Group 13 — the boron family — contains boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). Their valence configuration is ns2np1, so they have three valence electrons and a common oxidation state of +3. Boron is the only non-metal (a metalloid); the rest are typical metals whose metallic character increases down the group.
Going down the group the atomic radius increases, but not smoothly. Ga is actually slightly smaller than Al because the poor shielding by the intervening 3d electrons (the d-block contraction) lets the nucleus pull the outer electrons in more tightly. Ionisation enthalpy therefore decreases from B to Al but then behaves irregularly. A central feature is the growing stability of the +1 oxidation state on descending the group — the inert pair effect. For Tl the +1 state is actually more stable than +3, so Tl3+ is a strong oxidising agent. Aluminium is exclusively +3, gallium and indium show both, and thallium prefers +1.
Anomalous behaviour of boron. Boron differs sharply from the rest of the group because of its very small size, high ionisation enthalpy, high electronegativity and absence of d-orbitals in its valence shell. It is a hard, refractory non-metallic solid, forms only covalent compounds, and its maximum covalency is restricted to four (it cannot expand its octet, unlike Al which forms [AlF6]3-). Boron's compounds such as BF3 and BCl3 are electron-deficient Lewis acids. Boron resembles silicon (its diagonal neighbour) more than it resembles aluminium.
Important compounds. Borax, Na2B4O7·10H2O, is a white crystalline solid; in solution it is alkaline (it hydrolyses) and on strong heating gives a transparent borax bead of NaBO2 and B2O3, used to identify coloured metal ions. Orthoboric acid, H3BO3, is a weak monobasic Lewis acid (not a protonic acid): it accepts OH- rather than donating H+, forming [B(OH)4]-. It has a layered structure held by hydrogen bonds and feels soapy. Diborane, B2H6, is the simplest borane; it is electron-deficient and held together by two three-centre two-electron (3c-2e) ‘banana’ bonds in which two bridging hydrogens each bind to both boron atoms using only one electron pair for the three atoms.
Aluminium is the most abundant metal in the Earth's crust. It is strongly electropositive yet protected by a tough oxide film. It is amphoteric — it dissolves in both acids and alkalis: 2Al + 6HCl → 2AlCl3 + 3H2, and 2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2. Al2O3 and Al(OH)3 are likewise amphoteric. Aluminium is used in aircraft, electrical cables, packaging and in the thermite welding reaction (it reduces Fe2O3).