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Hydrocarbons are compounds containing hydrogen and carbon atoms only. The alkanes are a homologous series of hydrocarbons with the general formula CnH2n+2.

The models show the first four unbranched members of this series. You can also see the structural and displayed formulae for these alkanes.

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chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

methane, CH4

ethane, C2H6

propane, C3H8

butane, C4H10

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 group

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 alkanes is most easily shown as —CH3.

General formula

A general formula is a chemical formula used to represent the members of a homologous series. The general formula for the alkanes is CnH2n+2.

Hexane molecules, for example, contain six carbon atoms:

n = 6
2n + 2 = (2 × 6) + 2 = 14

So the molecular for hexane is C6H14.

Structural and displayed formulae

Name of alkane
Structural formula
Displayed formula
methane CH4 displayed formula of methane
ethane CH3CH3 displayed formula of ethane
propane CH3CH2CH3 displayed formula of propane
butane CH3CH2CH2CH3 displayed formula of butane