The first ionisation enthalpies of four elements Be , B , N and O are 801 , 899 , 1314 and 1402 kJ mol ⁻¹ (not necessarily in that order). The correct assignment of the first ionisation enthalpies to the respective elements is:

Step-by-step solution

The first ionisation enthalpy generally increases across a period from left to right due to an increase in effective nuclear charge. However, there are exceptions due to the extra stability of fully-filled and half-filled subshells. For the given elements, Be ( 1s^2 2s^2 ) has a fully-filled 2s subshell, making its first ionisation enthalpy higher than that of B ( 1s^2 2s^2 2p^1 ). Similarly, N ( 1s^2 2s^2 2p^3 ) has a stable half-filled 2p subshell, making its first ionisation enthalpy higher than that of O ( 1s^2