Chemguide

Extra material needed for the 2010 exam - Chapter 18

This page lists the material from the old Edexcel syllabus which isn't covered by my new Edexcel IGCSE Chemistry book.

Shapes of alkanes and alkenes

There are two minor additions here to make more explicit the arrangements of the bonds around a carbon atom with four single bonds (in alkanes, for example), or with two single and one double bond (as in the alkenes, for example). There are two syllabus statements that describe the arrangements in geometric terms:

• "recall that, in alkanes, the four bonds on each carbon atom are directed to the corners of a tetrahedron"

• "recall that, in alkenes, the bonds on each carbon atom are directed to the corners of an equilateral triangle"

Look at the diagrams in the bottom left-hand corner of page 150.

First, the four single bonds, as in an alkane:

This arrangement is described as tetrahedral. A tetrahedron is a regular triangular-based pyramid. The carbon atom is at the centre of the tetrahedron, and the four bonds are directed towards the four corners of the tetrahedron.

That's what gives methane its shape - see the simple diagram of a methane molecule at the top of page 156.

Secondly, the arrangement of the bonds around a doubly-bonded carbon atom:

The two single bonds and the double bond are all in the same plane. In this case, you can think of the carbon atom as being at the centre of an equilateral triangle, with the bonds directed towards the corners of the triangle.

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