p-Block Elements (Class 11): Groups 13 and 14

The two organising ideas

Almost every question in these two groups is an application of one of two principles, so learn these rather than a list of facts.

1. The inert pair effect

Going down a group, the outermost s electrons become increasingly reluctant to participate in bonding. The result is that the lower oxidation state — +1 in group 13, +2 in group 14 — becomes progressively more stable relative to the group state.

So at the top of group 13 the +3 state dominates, while at the bottom the +1 state is the stable one. The same pattern holds in group 14 with +4 and +2. Any question asking which oxidation state is more stable for a heavier element is answered by this effect.

2. Anomalous behaviour of the first element

Group 13

Boron is a non-metal; the rest are metals

Boron's small size and high ionisation energy make it behave as a non-metal, forming covalent compounds. The elements below it are metallic and form ionic compounds in their common states. Metallic character increases down the group, as it does in every p-block group.

Electron deficiency

Boron compounds of the form BX₃ have only six electrons around boron, so they are electron deficient and act as Lewis acids. This is why boron trihalides accept a lone pair readily, and it underlies the whole chemistry of boranes.

The order of Lewis acidity among the boron trihalides is a classic question, and it runs opposite to what electronegativity alone suggests — because back-donation from halogen lone pairs into boron's empty p orbital is strongest for the smallest halogen, reducing its acidity most.

Compounds worth knowing

• Diborane, with its two three-centre two-electron bridge bonds, which cannot be drawn conventionally.
• Borax, and the borax bead test used to identify metal ions by the colour of the bead formed.
• Boric acid, which is a weak monobasic acid — and, importantly, it acts as an acid by accepting hydroxide rather than donating a proton. That mechanism is regularly asked.
• Aluminium chloride, which exists as a dimer in the vapour, with chlorine bridges completing the octet at aluminium.

Amphoteric behaviour

Aluminium oxide and hydroxide react with both acids and bases, forming aluminium salts with acids and aluminate ions with bases. Demonstrating amphoterism with both equations is a standard answer.

Group 14

Catenation

Carbon forms long chains and rings with itself far more readily than any other element, because the C–C bond is strong and carbon is small. Catenation ability decreases sharply down the group as bond strength falls, which is why silicon chains are limited and heavier members barely catenate at all.

This single property is why organic chemistry exists as a subject, and it is the standard justification asked for in a comparison question.

Carbon's inability to expand its octet

Carbon has no d orbitals, so it is limited to four bonds. Silicon can use its d orbitals and form six-coordinate species. This explains why carbon tetrachloride does not hydrolyse in water while silicon tetrachloride does — silicon can accept the attacking water molecule, carbon cannot. It is one of the most frequently asked comparisons in the chapter.

Allotropes of carbon

Graphite's conduction along the layers but not across them, and its lubricating property from weak interlayer forces, are the two most-asked features.

Silicates and silicones

Silicates are built from tetrahedral units sharing corners, and the extent of sharing determines whether the structure is discrete, chain, sheet or three-dimensional. Silicones are synthetic polymers with alternating silicon and oxygen backbones and organic side groups, valued for water repellency and thermal stability.

Frequently asked questions

Why does the inert pair effect strengthen down a group?

Because poor shielding by intervening d and f electrons increases the effective nuclear charge on the s electrons, holding them more tightly and making them less available for bonding.

Why does carbon tetrachloride not hydrolyse but silicon tetrachloride does?

Carbon has no d orbitals and cannot accommodate an attacking water molecule. Silicon can expand its coordination using d orbitals, so hydrolysis proceeds readily.

Why is boric acid monobasic despite having three hydroxyl groups?

Because it does not donate its own protons. It accepts a hydroxide ion from water, releasing one proton in the process, so only one acidic equivalent results.

Why does catenation decrease down group 14?

Because the element–element bond becomes weaker as atomic size increases and overlap becomes poorer, so long chains are no longer stable.