First structure is CH₃-CH₃. It is an alkane with two carbon atoms hence it is called ethane.
Second structure is CH₃-CH₂-CH₂-CH₃. It is an alkane with four carbon atoms hence it is called butane.
Third structure is CH₄. It is an alkane with one carbon atom hence it is called methane.
Fourth structure is CH₃-CH₂-CH₃. It is an alkane with three carbon atoms hence it is called propane.
