As compared to its parent derivative, an ethyl radical contains

[Step 1]:
The general formulae of alkanes is  ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$
The alkyl general formulae is  ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+1}$
The alkyl group has one hydrogen less than alkane.
[Step 2]:
Therefore the ethyl radical contains one hydrogen less than ethane
Since ethane is C₂H₆
and ethyl is C₂H₅