Ionization Enthalpy

First ionization enthalpy is the energy needed to remove the most loosely bound electron from a gaseous atom, and although it generally increases across a period, there are anomalies due to subshell stability. Beryllium has the configuration 1s^2 2s^2, with a completely filled and especially stable 2s subshell, so removing an electron requires comparatively high energy. Boron has the configuration 1s^2 2s^2 2p^1, and its single 2p electron is at slightly higher energy and is better shielded, making it easier to remove than expected. Hence beryllium has the higher first ionization enthalpy despite boron's greater nuclear charge. The option citing boron's nuclear charge is true but is outweighed by the subshell stability effect. The option of more shells is false since both are in period two. Being a metal does not by itself raise ionization enthalpy. This anomaly is a classic NCERT periodicity example. Such questions reward conceptual clarity, since a student who truly grasps ionization enthalpy can solve many superficially different variants with the same approach. Understanding ionization enthalpy in this way ties directly into the wider study of classification of elements and periodicity in properties, where the same reasoning recurs across many problems. Plausibility check: similar fully filled stability explains why magnesium exceeds aluminium too, confirming the pattern.