# Why does the ketone start from propane, but not methane?

And thus neither $\text{formaldehyde, " "H"_2"C=O}$, nor $\text{acetaldehyde, " "H"_3"CC"("=O")"H}$ supports an internal carbonyl. Both functions are terminal, and thus these species are formally $\text{aldehydes}$. Capisce?
On the other hand, $\text{acetone}$, ${\text{H"_3"CC(=O)CH}}_{3}$, does support the internal carbonyl group, and can thus be classified as a $\text{ketone}$.