Why does methane exist as a gas and nonane exist as liquid​

Molecular weight

Explanation:

The difference in the physical state of nonane vs methane at STP.  Methane is a very small molecule with a molecular weight of 16 g/mol.  It exists as a gas at standard temperature and pressure.  Nonane is a much larger molecule with a molecular weight of 128.2 g/mol.  It exists as a liquid at standard temperature and pressure.  

All gasses at STP are Made of small molecular weight molecules.  The smaller a molecule, the more movement the individual atoms have.  The increased movement of smaller molecules allows them to have more space between individual molecules.  The increased molecular motion and space between molecules is one of the main differences between a gas versus a liquid or solid.  The larger nonane molecules have less molecular motion and so are can be closer together and so exist as a liquid instead of a gas.