# Why is the melting point of propane lower than that of both methane and ethane?

I've seen their properties in my textbook and multiple online sources, but I can't seem to find a straight answer as to why. Excluding propane, the trend is increased melting point as the molecule gets bigger, but propane is the weird exception here.

$$\begin{array}{c|c|c|c}\hline \text{Chemical} & n \text{ in }\ce{C_nH_{2n+2}} & \text{m.p. / K} & \text{m.p.}(n) - \text{m.p.}(n-1)\text{ / K} \\ \hline \text{Methane} & 1 & 90.7 & \\ \text{Ethane} & 2 & 90.4 & -0.3 \\ \text{Propane} & 3 & 85.5 & -4.9 \\ \text{Butane} & 4 & 133\mathrm{-}139 & +50.5 \\ \text{Pentane} & 5 & 144.1 & +8.0 \\ \text{Hexane} & 6 & 177\mathrm{-}179 & +36.9 \\ \text{Heptane} & 7 & 182.2\mathrm{-}183.0 & +4.6 \\ \text{Octane} & 8 & 216.0\mathrm{-}216.6 & +33.7 \\ \text{Nonane} & 9 & 219.0\mathrm{-}220.0 & +3.2 \\ \text{Decane} & 10 & 242.7\mathrm{-}243.9 & +23.8 \\ \hline \end{array}$$
• In general even number of $\ce{C}$ atoms in a chain are much more accommodating to a lattice than odd number of carbons. This is why the increase in boiling point from odd $\implies$ even is much more higher than even $\implies$ odd. Jul 14 at 9:58