= Alkenyl group
An <alkenyl group> is a monovalent carbon–hydrogen substituent derived from an <alkene> by removing one <hydrogen> atom. The attachment can be at a double-bonded <carbon> or elsewhere in the carbon skeleton. For an acyclic parent with one <double bond>, the group composition is $\mathrm{C_nH_{2n-1}}$.<Alkenyl group in ChEBI>[CHEBI:37614; definition and named examples]{ref}
For example, a <vinyl group> attaches as $\mathrm{R-CH=CH_2}$ and contributes $\mathrm{C_2H_3}$, whereas an <allyl group> attaches as $\mathrm{R-CH_2-CH=CH_2}$ and contributes $\mathrm{C_3H_5}$. The attachment atom in the first example belongs to the <double bond>; in the second it is adjacent to the <double bond>. A <prenyl group> is another <alkenyl group>, with five <carbon> atoms.<Alkenyl group in ChEBI>[Ontology; vinyl, allyl and prenyl subclasses]{ref}
The missing hydrogen identifies an attachment site in a larger <molecule>; it does not mean that the completed attached group must contain an unpaired <electron>. Formulae alone also do not specify connectivity. For $n=3$, the composition $\mathrm{C_3H_5}$ does not by itself tell where the attachment or <double bond> lies. A structural formula supplies information that the atom count omits.
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