Classification of Elements and Periodicity in Properties — Predicted 2026
AI-Predicted Question Angles for UPSC 2026
Comparison of Ionization Enthalpies with Exceptions
highNEET consistently tests the exceptions to the general trend of increasing ionization enthalpy across a period. Specifically, questions comparing $IE_1$ of Group 2 vs Group 13 (e.g., Be vs B, Mg vs Al) and Group 15 vs Group 16 (e.g., N vs O, P vs S) are highly probable. The reasoning behind these exceptions (stable $s^2$ or $p^3$ configurations) is a key concept that students often overlook, making it a good differentiator. Expect questions asking to identify the correct order or explain the deviation.
Electron Gain Enthalpy Trends and Anomalies
highSimilar to ionization enthalpy, electron gain enthalpy trends and their exceptions are frequently examined. The anomaly where second-period elements (like F, O) have less negative electron gain enthalpies than their third-period counterparts (Cl, S) due to electron-electron repulsion in smaller shells is a prime candidate for questions. Additionally, the positive electron gain enthalpy of noble gases and alkaline earth metals is a common conceptual trap. Questions will likely involve ordering elements by their electron gain enthalpy or explaining specific anomalies.
Isoelectronic Species and Lanthanoid Contraction
mediumThe concept of isoelectronic species and their ionic radii is a fundamental aspect that tests a student's understanding of effective nuclear charge. Questions asking to arrange a series of isoelectronic ions in order of increasing/decreasing size are very common. Furthermore, Lanthanoid Contraction, its causes (poor 4f shielding), and its consequences (similar radii of 4d and 5d elements, making their separation difficult) are important advanced concepts that appear periodically. While not as frequent as the main periodic trends, these topics are considered high-value for testing deeper understanding.