Classification of Elements

Syllabus Unit: Inorganic Chemistry

Why it matters

Chapter overview

The periodic table is the single most-tested organizing framework in JEE inorganic chemistry: once you understand why properties repeat with atomic number, atomic/ionic radii, ionisation enthalpy, electron gain enthalpy, electronegativity and metallic character all fall out as consequences of effective nuclear charge and shell structure, rather than as facts to memorise separately.

Chemistry Syllabus Structure Inorganic Chemistry ~33% weightage

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s-BlockGroups 1-2, ns^1-2

p-BlockGroups 13-18, ns^2np^1-6

d-BlockGroups 3-12, transition metals

f-BlockLanthanoids & actinoids

1The Periodic Table & Periodicity

Moseley's Modern Periodic Law replaced Mendeleev's mass-based ordering with atomic number, giving the 18-group, 7-period table whose s/p/d/f blocks predict an element's chemistry from its electronic configuration alone.

Learning Objectives

State the Modern Periodic Law and explain how Moseley's X-ray data corrected Mendeleev's atomic-mass ordering.
Identify the s, p, d and f blocks of the periodic table from an element's electronic configuration.
Explain why periods 6 and 7 contain 32 elements each due to the filling of f-orbitals.
Locate representative, transition and inner-transition elements by their group/period position.

Key Terms

Modern Periodic LawPhysical and chemical properties of elements are periodic functions of their atomic number.

Block (s/p/d/f)Classification of an element by the subshell that receives its last (differentiating) electron.

PeriodA horizontal row; period number equals the highest principal quantum number (n) occupied in the ground state.

GroupA vertical column whose members share the same valence-shell electronic configuration pattern.

Mendeleev arranged elements by increasing atomic mass and left gaps for undiscovered elements, but this ordering produced anomalies (e.g. Ar before K despite Ar's higher mass). Moseley's 1913 X-ray spectroscopy showed nuclear charge, not mass, drives periodicity, giving the **Modern Periodic Law**.

Structure of the long-form table

7 periods (rows), numbered by the valence shell's principal quantum number n.
18 groups (columns); Groups 1-2 and 13-18 are representative (s- and p-block), 3-12 are transition (d-block).
Periods 6 and 7 hold 32 elements each because the 4f/5f (lanthanoid/actinoid) series is inserted between the d-block's group 3 and group 4.
The f-block (lanthanoids, actinoids) is conventionally placed below the main table for width.

Exam TipJEE frequently tests block identification from configuration: an element ending in ...(n-1)d^x ns^2 is d-block; one ending in ns^2 np^x is p-block; ...(n-2)f^x is f-block.

Why properties repeat

Periodicity arises because valence-shell electronic configurations repeat as atomic number increases — elements in the same group have the same outer-shell pattern (e.g. ns^1 for alkali metals), so they show similar chemical behaviour despite very different atomic masses.