1.8 Carbon's Other Geometries

Carbon is tetrahedral when it has only single bonds, but it also has other geometries when double or triple bonds are introduced. We already saw that in acetylene, the atoms are all arranged in a straight line. It's a linear molecule, and triple-bonded carbon has linear geometry.

Carbon with one double bond (and two single bonds) has trigonal planar geometry. The bonds are arranged in a flat plane, with 120° angles between them, like an equilateral triangle. A good example is benzene C₆H₆, consisting of a ring of six carbon atoms surrounded by six hydrogens.

When a carbon atom has two double bonds, its geometry becomes linear. Carbon dioxide or CO2 has a molecular structure with one central carbon atom double-bonded to two oxygen atoms. Because the geometry is linear, the atoms are arranged in a straight line: C=O=C, where = represents a double bond.

Nitrogen and oxygen atoms also change their geometries in the same ways when they are double-bonded or triple-bonded, although we don't always see it in the case of oxygen since it only takes two bonds.

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