C1aL17  Alkanes

Key Words

Alkane - Compound that consists of hydrogen and carbon in which the carbons are linked by single bonds.

Covalent - Bonds that share electrons.

Hydrocarbons - compounds consisting of hydrogen and carbon.

Isomers - compounds that contain the number of each atom, but are different structures.

Methane - CH₄, natural gas.

Grade E

Hydrocarbons consist of carbon and hydrogen only.  Alkanes are a family of hydrocarbons in which all the carbon atoms are linked together by single bonds between the carbon atoms, like this:

In the middle, each carbon has 2 H atoms.  At each end the carbons have 3 H atoms.

Methane is the simplest alkane.  Its molecular formula is CH₄.  Its atomic model is like this:

The general formula for alkane molecules is:

C_nH_2n+2

If there are 5 carbon atoms, the formula is:

C₅H₁₂

Grade C

The atoms in alkanes are held by strong covalent bonds where the electrons are shared as shown in the Lewis Diagram:

All alkanes have similar properties.  They all have a name that ends in "-ane".  You can use this mnemonic to help you to remember the first six:

Mr Everett punishes bad pupils harshly

Methane, ethane, propane, butane, pentane, hexane

CH₄,   C₂H₆,     C₃H₈,     C₄H₁₀,    C₅H₁₂,   C₆H₁₄

Grade A

In the cases of methane, ethane, and propane, it is possible to have only one layout of the molecule.  This changes with butane.  We can have two different molecules that share the formula C₄H₁₀.  This molecule is butane:

This molecule is methyl propane:

The two molecules have the same numbers of each atom.  They have the same molecular formula.  However the display formulae are completely different, as are their properties.  They are called isomers.  Methyl propane is an isomer of butane.

The longer then chain, the more isomers there are.