If you heat your home with natural gas, you are burning mostly methane. Ethane is the second smallest hydrocarbon. Each molecule has two carbon atoms holding onto each other, and each has three hands left over to hang on to three hydrogen atoms. Ethane is a little heavier than methane, but it's still light enough to be a gas. Some of our natural gas is ethane.
Propane has three carbon atoms lined up in a row.
The two at the ends hold on to three hydrogen atoms each, but the one in the middle can only hold two hydrogens. Propane is heavier than methane and ethane. Some cars and trucks run on propane. If a molecule has more than three carbon atoms, it gets slower and heavier. Butane has four carbon molecules, so it's heavier than propane.