Lesson 3: Molecules in 3D

Flat triangles and straight lines

Section 3 of 4

Not every carbon has four separate bonds. A double or triple bond still points one way, so it changes the shape.

To work out a shape, count the directions around the middle atom. A direction is a bond or a lone pair, and a double or triple bond counts as one direction. Carbon has no lone pairs in these molecules.

Here is a rule to keep the two kinds of counting apart: for counting bonds, count lines; for shape, count directions.

  • 4 directions: a tetrahedron, 109.5° apart, as in methane.
  • 3 directions: a flat triangle, about 120° apart, as at each carbon in ethene. All six atoms of ethene lie flat in one plane.
  • 2 directions: a straight line, 180° apart, as in ethyne and carbon dioxide.
Shapes made by 4, 3 and 2 directions, and bent water Four panels. 4 directions around a carbon point to the corners of a tetrahedron, 109.5 degrees apart, as in methane. 3 directions make a flat triangle about 120 degrees apart, as at each carbon in ethene; the double bond counts as one direction. 2 directions make a straight line, 180 degrees, as in carbon dioxide. Water's oxygen has 2 bonds and 2 lone pairs, 4 directions in all, so its two bonds are bent at 104.5 degrees. 4 directions tetrahedron, 109.5° methane 3 directions flat triangle, about 120° a carbon in ethene 2 directions straight line, 180° carbon dioxide 2 bonds + 2 lone pairs bent, 104.5° water
4, 3 and 2 directions make a tetrahedron, a flat triangle and a straight line. Water's oxygen has 4 directions too, but two of them are lone pairs.
Ethene, C₂H₄Two carbon atoms joined by a double bond, each also bonded to two hydrogen atoms.HHCCHH
Ethene is flat: each carbon has 3 directions, about 120° apart.
Carbon dioxide, CO₂One carbon atom joined by double bonds to two oxygen atoms, in a straight line.OCO
Carbon dioxide is straight: the carbon has 2 directions, 180° apart.

Quick check

A carbon atom has one triple bond and one single bond. What shape do its bonds make?