Why are small linear hydrocarbons gasses?
Because dispersion forces INCREASE with INCREASING chain length.
Boiling point for hydrocarbons largely depends on dispersion forces between the hydrocarbyl chains. The longer the chain, the greater the dispersion force between chains, and the greater the boiling point.
The first 4 hydrocarbons, methane, ethane, propane, and butane, are room temperature gases, and their boiling points are respectively
They have lower boiling points due to weaker intermolecular forces.
London forces (also known as Van der Waals forces or induced dipole-dipole interactions) is the term given to the weak intermolecular forces that exist between all molecules, whether polar or non-polar. They act between induced dipoles in different molecules.
The electrons in a molecule can be in any location in their shell at any given point in time. At any instant, an instantaneous dipole will exist, but it's position is constantly shifting. (That is to say, there will be more electrons on one side of the molecule than on the other side of the molecule, but this imbalance of electrons keeps changing).
The instantaneous dipole induces a dipole on a neighbouring molecule, and the induced dipole induces further dipoles on neighbouring molecules, which then attract one another.
The more electrons in each molecule, the larger the instantaneous and induced dipoles, the greater the induced dipole-dipole interactions and the stronger the attractive forces between the molecules.
Small linear hydrocarbons have a small surface area, and so less surface area contact is possible between molecules. The London forces between the molecules will be smaller and so less energy is required to overcome the forces and turn them into gasses.