Methane and ethane

You know that biogas, made from the waste of livestock farms, is used as a fuel. Its main fuel component is methane gas. Methane is also found in the marsh gas given off as organic matter decays in swamps. It is the simplest hydrocarbon, with the formula CH₄.

When crude oil wells are drilled, a gas called ethane is released from them. Ethane is also a hydrocarbon; its formula is C₂H₆, and its structure is drawn below.

Look at the methane and ethane molecules. In methane there are only bonds between carbon and hydrogen atoms. In ethane there are bonds between carbon and hydrogen atoms and also between the two carbon atoms.

Why every carbon has four lines

Each line in a structure is one covalent bond — a shared pair of electrons, as in the Lewis structures of grade 10. Carbon needs four more electrons to fill its outer shell, so it always forms four bonds; hydrogen needs one, so it always forms one. Check any alkane and you will find four lines at every C and one at every H. In ethane, one of each carbon's four bonds is used to hold the other carbon, so each has three hydrogens instead of methane's four. That simple count is how the formulas of the whole family are built.