Class XI Chemistry (CBSE – 2025–26 Academic Session)

Section A – Some Basic Concepts of Chemistry

This section builds the foundation of Chemistry. It explains what Chemistry is and how matter is classified. Matter is anything that has mass and occupies space. It is broadly classified into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).

The chapter explains physical and chemical properties of matter and introduces SI units for measurement. Concepts like precision, accuracy, significant figures, and scientific notation help students understand how measurements are expressed correctly in scientific work.

Important laws of chemical combination are discussed, such as:

Law of Conservation of Mass

Law of Definite Proportions

Law of Multiple Proportions

Gay Lussac’s Law of Gaseous Volumes

Avogadro’s Law

Dalton’s Atomic Theory is introduced, explaining the basic nature of atoms. The section also explains atomic mass, molecular mass, formula mass, mole concept, percentage composition, empirical formula, molecular formula, and limiting reagent.

The mole concept connects microscopic particles to measurable laboratory quantities, which is essential for solving numerical problems in Chemistry.

Questions – Section A

Explain the Law of Conservation of Mass with an example.

What is the difference between empirical formula and molecular formula?

Define mole concept and explain its importance in chemical calculations.

What are significant figures? State the rules for determining them.

What is a limiting reagent? Why is it important in reactions?

Section B – Structure of Atom

This section explains the internal structure of the atom. It begins with Dalton’s atomic theory and discusses its limitations after the discovery of subatomic particles.

J.J. Thomson discovered the electron through the cathode ray experiment and proposed the plum pudding model. Rutherford’s gold foil experiment proved the existence of a small, dense, positively charged nucleus.

Bohr’s atomic model introduced the idea that electrons move in fixed energy levels or orbits around the nucleus. According to Bohr, electrons can jump between energy levels by absorbing or emitting energy in the form of radiation.

The hydrogen spectrum and Rydberg formula explain the line spectrum observed in hydrogen.

Further developments led to the Quantum Mechanical Model, which describes electrons as waves rather than particles moving in fixed paths. Important concepts include:

de Broglie’s wave-particle duality

Heisenberg’s Uncertainty Principle

Schrödinger’s wave equation

The four quantum numbers (n, l, m, s) describe the energy, shape, orientation, and spin of electrons.

Rules for filling orbitals include:

Aufbau Principle

Hund’s Rule

Pauli’s Exclusion Principle

This section helps students understand atomic stability and electron configuration.

Questions – Section B

Explain Rutherford’s gold foil experiment and its conclusions.

State the postulates of Bohr’s atomic model.

What is Heisenberg’s Uncertainty Principle?

Define the four quantum numbers and explain their significance.

State and explain Aufbau principle and Hund’s rule.

Section C – Classification of Elements and Periodicity in Properties

This section explains the development of the periodic table. Mendeleev’s Periodic Law was based on atomic mass, while the Modern Periodic Law is based on atomic number.

Elements are classified into:

s-block elements

p-block elements

d-block elements (transition elements)

f-block elements (inner transition elements)

The section discusses periodicity, which is the repetition of similar properties at regular intervals.

Important periodic properties include:

Atomic radius

Ionic radius

Ionization energy

Electron affinity

Electronegativity

Atomic radius decreases across a period due to increased nuclear charge, while it increases down a group due to additional shells.

Covalent radius, van der Waals radius, and metallic radius are also explained.

This section helps students understand how electronic configuration determines chemical behavior and reactivity.

Questions – Section C

State the Modern Periodic Law.

Differentiate between s-block and p-block elements.

Why does atomic radius decrease across a period?

What is ionization energy? How does it vary in the periodic table?

Explain the difference between covalent radius and van der Waals radius.