Compare nuclear charge

• Beryllium: atomic number $Z=4$
• Boron: atomic number $Z=5$

Since nuclear charge depends on atomic number, beryllium has lesser nuclear charge than boron.

Compare first ionisation enthalpy

Electronic configurations:

• Be: $1s^2 2s^2$
• B: $1s^2 2s^2 2p^1$

The electron removed from:

• Be comes from the filled $2s$ subshell
• B comes from the $2p$ subshell

A filled $2s$ subshell is relatively more stable, and a $2p$ electron is easier to remove than a $2s$ electron.

Therefore, $IE_1(\text{Be}) > IE_1(\text{B})$

So beryllium has greater first ionisation enthalpy than boron.

Match with the options

• A: lesser nuclear charge and greater first ionisation enthalpy ✅
• B: greater nuclear charge and greater first ionisation enthalpy ❌
• C: greater nuclear charge and lesser first ionisation enthalpy ❌
• D: lesser nuclear charge and lesser first ionisation enthalpy ❌

Final answer

The correct option is A.