The Carbon Family: Quick Revision

A brief note on The Carbon Family. Read it before a test. Try each question at the end before you open its answer.

Group 14 in short

• Outer pattern $ns^2\,np^2$. States $+4$ and $+2$, with $+2$ more stable down the group.
• Covalent radius (pm): C 77, Si 118, Ge 122, Sn 140, Pb 146.
• First ionisation enthalpy (kJ/mol): C 1086, Si 786, Ge 761, Sn 708, Pb 715.
• E-E bond enthalpy (kJ/mol): C-C 348, Si-Si 297, Ge-Ge 260, Sn-Sn 240. Catenation falls.
• Carbon: no d orbitals, covalency at most 4, strong multiple bonds.

Allotropes of carbon

• Diamond: sp$^3$, 3D network, C-C 154 pm, 8 atoms in the cubic cell, density 3.51 g/cm$^3$, does not conduct.
• Graphite: sp$^2$, layers $335\ \text{pm}$ apart, C-C 141.5 pm, conducts along the layers, slippery.
• C$_{60}$: 60 atoms, 12 pentagons, 20 hexagons, 90 bonds, 30 double bonds. Made by an arc between graphite rods in helium, extracted with benzene.
• A cage with $n$ atoms has $n/2 - 10$ hexagons and $3n/2$ bonds.

Oxides of carbon

• CO: triple bond, 112.8 pm, neutral, poisonous, reducing. $\text{HCOOH} \to \text{CO} + \text{H}_{2}\text{O}$ with hot concentrated sulphuric acid. Producer gas is CO and nitrogen, and water gas is CO and hydrogen.
• CO$_2$: linear, 116 pm, no dipole, acidic. $\text{CaCO}_{3} + 2\text{HCl} \to \text{CaCl}_{2} + \text{H}_{2}\text{O} + \text{CO}_{2}$. Limewater turns milky, and clears with excess.
• Sodium hydrogencarbonate loses CO$_2$ at a low temperature and calcium carbonate at about 1200 K.

Silicon compounds

• Silica: a network of SiO$_4$ tetrahedra. Reacts with HF and hot alkali.
• Silicates: tetrahedra share $s$ corners, with $4 - s/2$ oxygens and the charge $4 - s$ for each silicon. $s$ = 0, 1, 2, 2.5, 3, 4 for orthosilicate, pyrosilicate, chain or ring, double chain, sheet, framework.
• Silicones: $(\text{CH}_{3})_{2}\text{SiCl}_{2}$ from Si and $\text{CH}_{3}\text{Cl}$ with Cu at 573 K, hydrolysed and condensed. Si-O-Si chains with methyl groups.
• Zeolites: aluminosilicate frameworks with exchangeable ions. Used to soften water and as sieves and catalysts.

A worked example

How many hexagons and bonds are in C$_{50}$? (A cage exists for 50 atoms.)

1. Bonds. $3 \times 50 / 2 = 75$.
2. Hexagons. $50/2 - 10 = 15$, and the faces are $12 + 15 = 27$. Euler: $50 - 75 + 27 = 2$.

Practice with new numbers

A program to try

Mistakes to avoid

• Saying that graphite is hard or that diamond conducts.
• Giving every fullerene 12 hexagons. It always has 12 pentagons.
• Calling CO an acidic oxide. It is neutral. CO$_2$ is acidic.
• Forgetting that a shared oxygen counts half in a silicate.

Check yourself

1. Why does graphite conduct and diamond not?

2. How many hexagons are on a cage of 60 atoms?

3. What is the charge on a sheet silicate for each silicon?

4. Name the monomer of a silicone chain and the group that ends it.

Sources

• OpenStax, Chemistry 2e, Chapter 18.
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition).
• Housecroft and Sharpe, Inorganic Chemistry, the chapter on the group 14 elements.