Following question on ketones?

The structure of ketone is R₁-CO-R₂ where R₁ and R₂ must be alkyl groups (hydrocarbon chains). If at least one of R₁ and R₂ is replaced by H atom, the molecule is aldehyde, but not ketone. i.e.
H-CO-H is methanal (an aldehyde), but not methanone.
CH₃-CO-H is ethanal (an aldehyde), but not ethanone.

No, what it means is that those terms are not the accepted nomenclature for the compounds you want them to represent. Ketones have the placement of carbon to either side of the C=O pair (the carbonyl functional group). They are aldehydes if the C=O pair is terminal to a chain.

Under the IUPAC Nomenclature (Naming system) is the carbonyl group (C=O) is on the end of the carbon chain then it is an ALDEHYDE (R-CHO) (R-C(=O)-H)
If the Carbonyl functional group in on a carbon in the chain then it is a KETONE (R-C(=O)-R' )

Since Methane & Ethane have only one & two carbon respectively they have no internal carbons in a chain, Hence they are NOT Ketones, ,but can only be Aldehydes.