Alkanes are saturated hydrocarbons (this means that there are NO double bonds in the molecule). They contain Hydrogen and Carbon atoms only. There are an immense number of possibilities. The simplest being methane with just ONE atom of Carbon.
You should be able to see ONE atom of Carbon and FOUR atoms of Hydrogen.
In ethane there are TWO atoms of Carbon and SIX atoms of Hydrogen.
In propane there are THREE atoms of Carbon and EIGHT atoms of Hydrogen.
In butane there are FOUR atoms of Carbon and TEN atoms of Hydrogen.
Perhaps you can work out the pattern now. If you multiply the number of Carbon atoms by 2 and then add 2 you get the number of Hydrogen atoms in the molecule. Here is a general formula for the alkanes:
The smallest of these chemicals are gases at room temperature: methane, ethane and propane. Larger molecules are liquids: butane, pentane, hexane, heptane, octane, etc.. Hexadecane (with 16 atoms of Carbon) melts at 18 degrees celsius. The very largest ones are solids. They are all flammable.
Alkenes are a very similar group of chemicals. They have a double bond between two Carbon atoms, they are “unsaturated” hydrocarbons. Their names follow a similar pattern: ethene, propene, butene, pentene, etc..
It is not possible to have “methene” because there have to be TWO atoms of Carbon in a molecule of an alkene. Think about it. Here is a general formula for the alkenes:
By now you will have realised that the spelling of chemical names is very important.