Answer:
- Because alkenes and alkynes are defined as molecules with at least one double (in the case of alkenes) or triple (in the case of alkynes) bond between two carbon atoms, while alkanes have only single bonds between any pair of carbon atoms.
Explanation:
Alkenes and alkynes are unsaturated molecules, because they have, at least, two adjacent carbon atoms bonded together by either a doubler or a triple bond: - C = C - or - C ≡ C -.
Hence, at least two carbon atoms are needed to form those double or triple bonds, while alkanes have only single bonds. The example of alkane with only one carbon atom is methane: CH₄, which is the most simple alkane.
The most simple alkene is CH₂ = CH₂, and the most simple alkyne is CH≡CH.
As you see, the very definition of alkenes and alkynes forces that those molecules must have at least two carbon atoms.
