Periodic Table Trends: JEE Main Complete Guide

Periodic Table Trends is a foundational chapter in JEE Main inorganic chemistry that underpins all subsequent group chemistry study. Questions from this chapter appear 1–2 times per session, often in the form of "arrange these elements in increasing order of X" or "which element has the highest Y." The chapter rewards students who understand the underlying principles — nuclear charge, shielding, electron configuration — rather than those who simply memorise order tables. This guide covers all major trends with the detailed reasoning that allows you to answer novel arrangement questions without memorisation.

Atomic Radius: Covalent, Ionic, and Van der Waals

Covalent radius: half the distance between two identical atoms bonded covalently. Ionic radius: measured from electron density map — cations are smaller than parent atoms (lost electrons, decreased repulsion), anions are larger (gained electrons, increased repulsion). Van der Waals radius: half the distance between non-bonded identical atoms in contact — largest of the three. Trend: atomic radius decreases across a period (increasing nuclear charge, same principal quantum number) and increases down a group (new electron shells added). The shielding effect: inner electrons shield outer electrons from nuclear charge; effective nuclear charge Z_eff = Z − sigma (Slater's rules). Across a period, Z_eff increases, pulling electrons closer. Down a group, new shells are added faster than Z_eff increases, so size increases. For the broader inorganic chemistry context, see our p-Block Elements Guide where periodic trends directly explain chemical behaviour.

Anomalous trend: within group 13, the trend is Al less than Ga — gallium is smaller than aluminium despite being below it in the group. Reason: d-block contraction (3d electrons are poor shields). Similarly in period 6, lanthanide contraction makes 6th period elements similar in size to 5th period elements. These anomalies are tested directly in JEE Main: "which is the correct order: Al, Ga, In, Tl?" Answer: Al less than Ga less than In less than Tl (after the anomalous Ga minimum, size increases normally).

Ionisation Energy: Trends and Anomalies

First ionisation energy (IE1): energy to remove one electron from a gaseous atom. Trend: increases across a period (higher Z_eff binds electrons more tightly) and decreases down a group (outermost electron farther from nucleus, more shielded). Major anomalies: (1) IE1(Be) greater than IE1(B): boron's outer electron is in 2p (higher energy, easier to remove) while beryllium's is in 2s. (2) IE1(N) greater than IE1(O): nitrogen has half-filled 2p (extra stability due to exchange energy), making its electron harder to remove than oxygen's. These two anomalies are among the most tested trends in JEE Main inorganic chemistry. Know the exact configurations: Be = [He]2s², B = [He]2s²2p¹, N = [He]2s²2p³, O = [He]2s²2p⁴. Test your periodic trends knowledge on our JEE Main chemistry mock tests featuring yearly-pattern questions on element ordering.

Second ionisation energy (IE2) is always greater than IE1 for the same element — removing a second electron from an already positive ion requires more energy. But IE2 can be dramatically higher than IE1 if the second electron is removed from a complete shell (e.g., Na has IE2 much higher than IE1 because the second electron comes from the filled 2p shell). JEE Main tests: "for which element is the ratio IE2/IE1 largest?" Answer: the alkali metals (Na, K, Li) — their IE2/IE1 ratios are among the highest because the second electron comes from a complete inner shell.

Electron Affinity and Electronegativity

Electron affinity (EA): energy change when a neutral gaseous atom gains one electron. Trend: generally increases across a period and decreases down a group (similar reasoning to IE). Anomaly: EA(N) less than EA(C) because nitrogen's half-filled 2p shell (2p³) is stable — it resists gaining an electron that would destabilise the configuration. Similarly EA(Be) approximately 0 and EA(Mg) approximately 0 because the added electron would go to the 2p/3p orbital (higher energy than the filled 2s/3s shell). EA(F) less than EA(Cl): chlorine has higher EA despite being below fluorine in the group — fluorine's small size creates high electron-electron repulsion when adding an electron to the already compact 2p shell. Cl has a more spacious 3p shell that accommodates the incoming electron better. Electronegativity (Pauling scale): increases across period, decreases down group. Fluorine is the most electronegative element (4.0 on Pauling scale). Noble gases: not assigned electronegativity (they don't form bonds in classic chemistry).

Applications of electronegativity in bond polarity: if electronegativity difference > 1.7 (Pauling scale), the bond is considered ionic; below 1.7, covalent (with polarity proportional to the difference). This threshold is often tested in JEE Main questions about bond type. Fajans' rules (covalent character in ionic compounds): higher charge on cation, smaller cation size, larger anion size — all increase covalent character. These rules relate back to polarisation of anion by cation (ionic polarisability).

Periodicity in Properties of Elements

Metallic character: decreases across period, increases down group. Most metallic: Cs (francium excluded due to radioactivity). Most non-metallic: F. Diagonal relationship: Li resembles Mg, Be resembles Al, B resembles Si (properties intermediate between the diagonal neighbours). These relationships arise because moving right (increasing Z) has an opposite effect on properties from moving down (increasing size) — and the diagonal pair balance these two trends. JEE Main tests diagonal relationships specifically in the context of Li-Mg and Be-Al. For example, both Li and Mg form nitrides directly with N2 (Li3N and Mg3N2) — unlike other alkali metals. Both Be and Al form amphoteric oxides (BeO and Al2O3 both dissolve in acid and base). Register on our platform to access inorganic trend practice problems. Our premium subscription includes complete JEE Main inorganic chemistry question banks. For group properties where these trends manifest in actual chemical behaviour, see our s-Block Elements Guide covering alkali and alkaline earth metals.

Study tip: do not memorise periodic trends as isolated rules. Understand 3 underlying principles — nuclear charge, shielding, and electron configuration stability — and derive all trends from these principles. This approach handles anomalous questions (which ask specifically about exceptions) far better than rote memorisation of trend tables.