What is here?

Hydrocarbons are compounds containing hydrogen and carbon atoms only. The alkenes are a homologous series of hydrocarbons with the general formula CnH2n.

The models show unbranched members of this series with 2, 3 or 4 carbon atoms. You can also see the structural and displayed formulae for these alkenes.

Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.

atom labels
ball and stick model
space-filling model
perspective view

ethene, C2H4

propene, C3H6

but-1-ene, C4H8

but-2-ene, C4H8

Homologous series

A homologous series is a series of compounds which:

  • have the same functional group
  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties.

Functional groups

A functional group is an atom, group of atoms or a bond responsible for the characteristic chemical reactions of a substance. The functional group in alkenes is the double carbon–carbon double bond, C=C.

General formulae

A general formula is a chemical formula used to represent the members of a homologous series. The general formula for the alkenes is CnH2n.

Hexene molecules, for example, contain six carbon atoms:

n = 6
2n = (2 × 6) = 12

So the molecular formula for hexene is C6H12.

Structural and displayed formulae

A structural formula shows the number of atoms of each element in a molecule, and their positions in the molecule.

A displayed formula shows the same information as a structural formula, but in addition it shows the bonds between the atoms.

Name of alkaneStructural formulaDisplayed formula
The model and displayed formula of but-2-ene show trans-but-2-ene. This is one isomer of but-2-ene. The other isomer is cis-but-2-ene. Find out about cistrans isomers here.