3_4 Group 14 (IVA) Elements: Trends, Inert Pair Effect and Carbon's Catenation

Group 14 (IVA) contains carbon, silicon, germanium, tin and lead. All have four valence electrons (

ns^2np^2

), all form a hydride

\mathrm{MH_4}

, a tetrachloride

\mathrm{MCl_4}

and a dioxide

\mathrm{MO_2}

, and all can show a valency of four. Yet the top and bottom of the group are almost unrecognisable as relatives. Carbon is the basis of organic chemistry and of diamond, while lead is a soft, dense metal. The change from one to the other teaches two important ideas, the inert pair effect and catenation.

Trends Down the Group

Group 14 elements: selected properties

Property	C	Si	Ge	Sn	Pb
Configuration	[He]2s²2p²	[Ne]3s²3p²	[Ar]3d¹⁰4s²4p²	[Kr]4d¹⁰5s²5p²	[Xe]4f¹⁴5d¹⁰6s²6p²
1st ionization energy (kJ/mol)	1086	787	762	709	716
Covalent radius (pm, approx.)	77	117	122	140	146
Melting point (°C)	sublimes ≈ 3640	1414	938	232	327
Character	Non-metal	Non-metal (semiconductor)	Metalloid	Metal	Metal

As the atoms grow, the valence electrons are held less tightly and metallic character increases. Carbon and silicon form giant covalent structures with very high melting points. Germanium is a metalloid, a brittle semiconductor with a shiny appearance. Tin and lead are true metals with low melting points, because their metallic bonding is relatively weak. The oxides follow the same pattern:

\mathrm{CO_2}

and

\mathrm{SiO_2}

are acidic, while germanium, tin and lead give amphoteric oxides that react with both acids and alkalis.

The Inert Pair Effect

Carbon, silicon and germanium use all four valence electrons, so +4 is their normal oxidation state. Further down the group, +2 becomes increasingly common, because the two

ns^2

electrons hold back from bonding. This inert pair effect comes from two causes. The heavy atoms' s electrons are held strongly, partly because the d and f electrons beneath them shield poorly. Their bonds are also long and weak, so the energy gained by forming two extra bonds does not repay the cost of using the s pair.

• Tin: +2 and +4 are both common. Tin(II) compounds are reducing agents, because tin easily goes up to +4.
• Lead: +2 is the stable state. Lead(IV) compounds such as

  \mathrm{PbO_2}

  are strong oxidizing agents, because lead easily drops back to +2.

Where Carbon and Silicon Are Found

Carbon occurs free as diamond and graphite, and in impure forms as coal. Combined, it occurs as carbonate rocks such as limestone (

\mathrm{CaCO_3}

), dolomite and magnesite, as carbon dioxide in the air, and in every living thing. Silicon is never found free, yet by mass it accounts for roughly 28% of the crust, second only to oxygen. It occurs as silica (

\mathrm{SiO_2}

) in sand and in quartz varieties such as rock crystal and amethyst, and as the huge family of silicate minerals: feldspars, micas, clays, talc and zeolites.

Why Carbon Is Different: Catenation

Catenation is the ability of an element's atoms to bond to each other in chains and rings. Carbon is the champion, forming millions of compounds with long chains, branches and rings. That is why a whole branch of chemistry, organic chemistry, is devoted to it. Catenation falls steeply down the group, because the element–element bond gets longer and weaker:

Approximate single-bond energies (kJ/mol)

Bond	C–C	Si–Si	Ge–Ge	Sn–Sn
Energy	346	226	188	151

Silicon prefers to alternate with oxygen rather than bond to itself, because the Si–O bond (about 452 kJ/mol) is much stronger than Si–Si. Silicon's long chains are therefore Si–O–Si chains, as in silicates and silicones. Carbon also forms strong multiple bonds (C=C, C≡C, C=O), which the larger atoms below it rarely do.

Size also controls reactivity. Carbon in

\mathrm{CCl_4}

is small and completely surrounded by chlorine atoms, so water has no way in, and carbon tetrachloride does not react with water. Silicon is larger and can briefly hold a fifth group while a water molecule attacks it, so silicon tetrachloride fumes in moist air and hydrolyses completely:

Cl[Si](Cl)(Cl)Cl.O.O.O.O>>O[Si](O)(O)O.Cl.Cl.Cl.Cl

Other differences also come from carbon's small size and high electronegativity. Carbon is the only member to form gaseous oxides, its hydrides (the alkanes) are stable while silanes catch fire in air, and diamond is far harder than any other element of the group.

Tips for remembering

• Non-metal to metal in one column: C and Si are non-metals, Ge is a metalloid, and Sn and Pb are metals. Group 14 shows the biggest change in character of any group.
• Lead lazily keeps its s pair: the 6s² electrons are held tightly, so Pb²⁺ is stable and PbO₂ is a strong oxidizing agent.
• Short, strong C–C bonds make chains: catenation falls sharply down the group because the bonds get longer and weaker.