Periodic Table & Periodic Trends

Concept overview

The modern periodic table arranges elements by increasing atomic number and groups elements with similar valence-electron configurations into the same column. The result is the striking periodicity of chemical and physical properties across rows and down columns.

Six trends dominate JEE questions. Atomic radius decreases across a period (increasing effective nuclear charge pulls electrons in) and increases down a group (new shells added). Ionisation energy — the energy to remove the outermost electron — increases across a period and decreases down a group, with anomalies at the group 2/13 and 15/16 boundaries where electronic-configuration stability flips the trend. Electron affinity has the same broad direction but with more anomalies (nitrogen has a slightly positive electron affinity due to its stable half-filled 2p³ configuration).

Electronegativity — an atom's tendency to attract a bonded electron pair — is quantified on the Pauling scale (F = 4.0, most electronegative; Cs = 0.7, least). It increases across a period and decreases down a group. Metallic character mirrors the trend in reverse — most metallic at bottom-left, most non-metallic at top-right.

Diagonal relationships (Li-Mg, Be-Al, B-Si) appear because ionic radii and charge densities of these pairs happen to match. Anomalies of the second period — smaller size, lack of d-orbitals, higher electronegativity — explain why Be(OH)₂ is amphoteric, why fluorine has weaker bonds than expected, and why boron and beryllium form unusual covalent halides. All these anomalies are direct JEE Main question fodder.

The chapter also introduces the s, p, d and f blocks — the four regions of the periodic table defined by which subshell is being filled. This classification will be extended in the p-block and d-block chapters.

Key formulas & concepts

• Atomic radius trend: decreases across a period; increases down a group
• Ionisation energy trend: increases across; decreases down; anomalies at Be–B and N–O
• Electronegativity (Pauling): F = 4.0; O = 3.5; N = 3.0; C = 2.5; Cs = 0.7
• Diagonal relationships: Li–Mg, Be–Al, B–Si show similar chemistry
• Metallic character: increases down a group; decreases across a period
• Electron affinity anomalies: N slightly positive; F slightly less than Cl due to inter-electronic repulsion in small 2p

Common mistakes to avoid

• Assuming ionisation energy strictly increases across the period — the Be→B and N→O drops break the monotone trend.
• Confusing electron affinity with electronegativity — the first is an energy (kJ/mol), the second a scale value.
• Predicting Cl to have higher electron affinity than F — F has a smaller size and greater inter-electronic repulsion in 2p, so Cl actually has a higher electron affinity.
• Ignoring diagonal relationships — Be(OH)₂ is amphoteric like Al(OH)₃; Li has ionic behaviour closer to Mg than to Na.

JEE Main relevance

JEE Main asks 1–2 periodic-trends problems every paper — usually one direct trend-ordering question and one identifying an anomaly. All archetypes are pattern-recognition.

JEE Advanced relevance

Advanced problems combine trends with specific compound comparisons — for instance, predicting whether Be or Mg forms a more covalent chloride, or explaining anomalous solubility of Li salts. Diagonal relationships are frequently tested at the Advanced level.

Previous-year questions (PYQ)

Filter the PYQ bank by 'Periodic Table'. Focus on ordering questions in the first pass — the same six trends recycle every year with different element sets.