Class 11 Chemistry – Classification of Elements and Periodicity in Properties

CBSE 2026–27 | NCERT-aligned free study resource

Why Classification?

Classification organizes elements so their properties can be studied systematically. The modern periodic table is based on atomic number.

Modern Periodic Law

The physical and chemical properties of elements are periodic functions of their atomic numbers. The long form periodic table contains periods and groups and reflects electronic configuration.

Blocks

Elements are grouped as s-, p-, d- and f-block elements according to the subshell receiving the differentiating electron.

Periodic Trends

Atomic radius generally decreases across a period and increases down a group. Ionization enthalpy generally increases across a period and decreases down a group, with exceptions. Electron gain enthalpy and electronegativity show characteristic trends influenced by effective nuclear charge and atomic size.

Valency and Chemical Character

Valence-shell electronic configuration helps explain common valencies, metallic character, non-metallic character and oxide behaviour.

MCQs

1. Modern periodic table is arranged by: (A) atomic mass (B) atomic number (C) density (D) valency. Answer: B.
2. Atomic radius generally decreases across a period because effective nuclear charge increases. Answer: True.
3. The d-block is associated mainly with: (A) transition elements (B) noble gases (C) alkali metals only (D) halogens only. Answer: A.

Practice

1. Explain effective nuclear charge.
2. Discuss trends in atomic radius and ionization enthalpy.
3. Differentiate s-, p-, d- and f-block elements.

HOTS

Predict which of two elements in the same period is likely to have the greater ionization enthalpy and justify from electronic structure.

Revision

Atomic number → electronic configuration → block/group/period → effective nuclear charge → periodic trends.