Carbon and silicon each join up with oxygen, one atom to two oxygens. Carbon dioxide, CO₂, is a gas you breathe out. Silica, SiO₂, is quartz and sand.
In carbon dioxide, the carbon puts two of its hands on each oxygen, making two C=O double bonds in a straight line. That uses up all four of carbon's hands and both of each oxygen's, so every CO₂ is a small, complete molecule. The molecules pull on each other only weakly, so carbon dioxide is a gas. Cooled to −78 °C it freezes into dry ice.
A double bond's second shared pair sits in clouds above and below the bond. Silicon's big atoms overlap poorly there, so that pair holds very weakly. Silicon puts its four hands on four different oxygens, with a single bond to each. Each oxygen then uses its second hand on another silicon, and the bonds join up in every direction into one giant network: silica.
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Silica's formula, SiO₂, gives the share of atoms in the network: one silicon to every two oxygens. Its bonds run through the whole crystal, as in diamond, so melting it means breaking strong Si–O bonds.
| Property | Carbon dioxide | Silica |
|---|---|---|
| formula | CO₂ | SiO₂ |
| bonds | 2 double bonds | 4 single bonds per silicon |
| built as | small molecules | a giant network |
| at room temperature | a gas | a hard solid |
| stays solid up to | −78 °C | about 1,700 °C |
Quartz and sand
Crystals of silica are quartz, and much of the sand on beaches is grains of quartz worn from rock. Most of Earth's crust is made of silicates, networks of silicon and oxygen with metal atoms such as aluminium, iron and calcium held among them.
Living things breathe out their used carbon as carbon dioxide, a gas that spreads through the air and the oceans, where plants take it up again. Silicon's oxide is rock, and it stays put.