Why does the simplest ketone have three carbon atoms and no fewer?
A ketone is R2C=O; therefore, if we make R1 a CH3 group (the simplest) and the second R group a CH3 group, that makes 3 C atoms. If you take one of the R groups and try to make it smaller (like making it a H atom), one ends up with RCHO and that is an aldehyde, not a ketone.
The simplest ketone is known as acetone, which has the molecular formula C3H6O. It contains three carbon atoms and no fewer because the carbon atom in the ketone group, which is a carbonyl group (C=O), requires two additional carbon atoms to form a stable structure.
To understand why, let's first look at the structure of a ketone. In a ketone, the carbonyl group consists of a carbon atom double-bonded to an oxygen atom (C=O). The remaining two valence bonds of the carbonyl carbon are filled by other groups or atoms.
Now, if we have fewer than three carbon atoms in the ketone, we come across some issues. If we have only one carbon atom, it cannot form a stable bond with the oxygen atom, leading to a highly reactive and unstable compound. On the other hand, if we have two carbon atoms, one of the carbon atoms will have an unused bond since the oxygen atom fills one bond, leading to an unstable structure as well.
By having three carbon atoms, the carbon attached to the carbonyl group can form stable bonds with both the oxygen atom and the remaining carbon atom. This allows for a stable and balanced structure, resulting in a common and widely found ketone known as acetone.
So, to answer your question, the simplest ketone has three carbon atoms and no fewer because it requires two additional carbon atoms to form a stable and balanced structure for the carbonyl group.