Silicon, one row down

Section 2 of 5

One row below carbon in the periodic table sits silicon, the second most common element in Earth's crust. It has four hands too.

A silicon atom has 14 protons and 14 electrons: 2 in its first shell, 8 in the second and 4 in its outer shell. With 4 outer electrons, silicon makes 4 bonds, just as carbon does. Silane, SiH₄, is a silicon atom holding four hydrogens, which point to the corners of a tetrahedron, as in methane. Disilane, Si₂H₆, is two silicon atoms joined by a single bond, built like ethane.

Silane, SiH₄One silicon atom joined by single bonds to four hydrogen atoms, which point to the corners of a tetrahedron, as in methane.SiHHHH
Silane, SiH₄: silicon holding four hydrogen atoms, the silicon version of methane.
Disilane, Si₂H₆Two silicon atoms joined by a single bond, each also bonded to three hydrogen atoms, as in ethane.SiSiHHHHHH
Disilane, Si₂H₆: two silicon atoms joined by a single bond, the silicon version of ethane.

Bigger atoms, weaker bonds

Silicon's outer electrons sit in a third shell, further from the nucleus, so a silicon atom is bigger than a carbon atom. A Si–⁠Si bond is 0.235 nm long, half as long again as a C–⁠C bond at 0.154 nm. Its shared pair sits further from both nuclei and holds them together less tightly.

Carbon's bonds beside silicon's A bar chart of the energy needed to break each bond, carbon's bonds in one colour and silicon's in another, in pairs: C–⁠C 346 kJ, Si–⁠Si 226 kJ, C–⁠H 413 kJ, Si–⁠H 318 kJ, C–⁠O 358 kJ, Si–⁠O 452 kJ. Carbon's bonds to carbon, hydrogen and oxygen are all close in strength. Silicon's bond to silicon is the weakest bar, and its bond to oxygen is the strongest, twice as strong as its bond to silicon. carbon silicon C–⁠C 346 kJ Si–⁠Si 226 kJ C–⁠H 413 kJ Si–⁠H 318 kJ C–⁠O 358 kJ Si–⁠O 452 kJ energy to break each bond
Energy to break each bond, in kilojoules for 602,000 billion billion bonds. Carbon's bonds to carbon, hydrogen and oxygen are close in strength. Silicon's bond to silicon is the weakest here, and its bond to oxygen the strongest.

A Si–⁠Si bond takes about 226 kJ to break, two thirds of a C–⁠C bond, and silicon's bond to hydrogen is weaker than carbon's too. Silicon's bond to oxygen is the strongest in the chart, twice a Si–⁠Si bond. So silicon has a great deal of energy to gain by swapping its partners for oxygen.

Chains that catch fire

Silane catches fire the moment it meets air, with no flame or spark, and burns to silica and water: SiH₄ + 2 O₂ → SiO₂ + 2 H₂O. Disilane and the longer silicon chains do the same. In water with a trace of alkali, silane breaks down to silica and hydrogen gas.

Silicon pulls on shared electrons less than hydrogen does, so in silane the silicon carries δ+. It is a big atom too, with room round it for an oxygen from water, or from the hydroxide that alkali brings, to come close. The energy hill for these reactions is low, and they start at room temperature. A chain of silicon atoms with hydrogens round it, the silicon version of a hydrocarbon, lasts only where air and water are kept away.

Quick check

Why do silicon's chains turn so readily into bonds between silicon and oxygen?