3_5 Oxides of Carbon and Silicon: CO, CO₂ and the Silica Network

Carbon and silicon are neighbours in group 14, and both have four valence electrons. Their dioxides could hardly be more different, though. Carbon dioxide is a gas that you breathe out, while silicon dioxide is sand and quartz, which melts above 1600 °C. The difference comes from how each atom bonds to oxygen. This lesson looks at carbon monoxide, carbon dioxide and silica, building on carbon's preference for multiple bonds from the previous lesson.

Carbon Monoxide: A Triple Bond with a Twist

Carbon monoxide, CO, has the same number of electrons as

\mathrm{N_2}

and, like it, a triple bond. Two of the bonds are ordinary shared pairs. The third is a coordinate (dative) bond, in which oxygen supplies both electrons. Each atom ends up with a lone pair and a complete octet, written

:\mathrm{C{\equiv}O}:

. The bond is very short (113 pm) and very strong (about 1072 kJ/mol).

Oxygen is more electronegative than carbon, so you might expect a large dipole with oxygen negative. In fact the dipole moment is tiny, only about 0.11 D, with the small negative end on carbon. The dative bond pushes electron density back towards carbon and almost exactly balances oxygen's greater electronegativity. Carbon's lone pair is what lets CO bind to metals. That includes the iron in haemoglobin, where CO binds far more strongly than oxygen, which is why it is so poisonous.

Carbon monoxide forms when carbon burns in a limited supply of oxygen, or when carbon dioxide passes over red-hot carbon. It burns with a blue flame to form carbon dioxide, which makes it a useful fuel and a reducing agent in extracting metals:

[C-]#[O+].[C-]#[O+].O=O>>O=C=O.O=C=O

Carbon Dioxide: A Small Linear Molecule

In

\mathrm{CO_2}

, carbon forms a double bond to each oxygen, O=C=O. With no lone pairs on carbon, the molecule is linear, and the two C=O bond dipoles point in opposite directions and cancel, so

\mathrm{CO_2}

has no overall dipole moment. Each C–O distance is about 116 pm, which fits a double bond. Separate molecules attract each other only by weak London forces, so carbon dioxide is a gas. When cooled, it turns directly into a molecular solid, dry ice, which sublimes at −78 °C.

Carbon dioxide is an acidic oxide. It dissolves in water to form a little carbonic acid and reacts with alkalis to form carbonates. A third, minor oxide, carbon suboxide (

\mathrm{C_3O_2}

), is a linear molecule, O=C=C=C=O, of little practical importance.

Silicon Dioxide: A Giant Network

Silicon is much larger than carbon, and its 3p orbitals overlap poorly with oxygen's 2p orbitals, so Si=O double bonds are weak. Silicon instead forms four strong Si–O single bonds pointing to the corners of a tetrahedron. Each oxygen bridges two silicon atoms. These

\mathrm{SiO_4}

tetrahedra link through every corner in three dimensions, building a giant covalent network in which every atom is held by strong bonds. Each silicon has four oxygens and each oxygen is shared by two silicons, so the ratio is 1 : 2.

\mathrm{SiO_2}

is therefore the simplest formula of the network, not the formula of a separate molecule.

Why CO₂ and SiO₂ are so different

Feature	Carbon dioxide	Silicon dioxide
Bonding to oxygen	Two C=O double bonds	Four Si–O single bonds
Structure	Separate linear molecules	Giant 3D network of SiO₄ tetrahedra
Forces to overcome on melting	Weak London forces	Strong covalent bonds
State at room temperature	Gas	Hard solid (melts above 1600 °C)
Reaction with water	Dissolves, forms carbonic acid	Insoluble

Quartz is the common crystalline form. Other forms, such as tridymite and cristobalite, keep the same tetrahedra but arrange them differently, with different Si–O–Si angles. If silica is melted and cooled, it does not crystallize easily. Instead it forms vitreous silica (silica glass), a disordered network. Silica glass is very transparent, softens only above about 1500 °C, barely expands on heating, insulates electricity and resists chemicals. It is attacked only by hydrofluoric acid and by hot alkalis:

\mathrm{SiO_2(s) + 4HF(aq) \rightarrow SiF_4(g) + 2H_2O(l)}

This reaction is used to etch glass. Atoms at the surface of a silica grain have unused bonding capacity, which makes silica gel a good adsorbent and drying agent.

Tips for remembering

• CO has a triple bond, one of them borrowed: two bonds share electrons normally, and oxygen donates the third pair, which nearly cancels the expected dipole.
• Carbon double-bonds, silicon single-bonds: small carbon makes C=O to two oxygens (a CO₂ molecule), while large silicon makes four Si–O single bonds and builds a network.
• Silica fears only HF: the Si–O network resists almost every acid except hydrofluoric acid, because Si–F bonds are even stronger.