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The first ionisation enthalpy of boron is lower than that of beryllium mainly because

Aboron has a smaller nuclear charge than beryllium
Bthe $2 p$ electron of $B$ is higher in energy and less penetrating than the $2 s$ electron of $Be$
Cboron has a larger atomic radius than beryllium by a wide margin
Dboron has a half-filled $2 p$ subshell

Answer: B

In

B

(

2 s^{2} 2 p^{1}

) the electron removed is a

2 p

electron, which penetrates less and is shielded by the

2 s

electrons, so it is easier to remove than a

2 s

electron of

Be

. Nuclear charge of B is actually larger and its

2 p

subshell is not half-filled.

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