NCERT Solutions Class 9 Science Chapter 9 Atomic Foundations of Matter

The NCERT Solutions for Class 9 Science Chapter 9 Atomic Foundations of Matter help students master chemical bonding and formula writing for the CBSE Class 9 exams, and build a base for JEE and NEET later, as per the 2026-27 syllabus. This page solves all 15 questions from the Revise, Reflect, Refine exercise in plain, step-by-step language. Each answer names the concept, shows every step, and adds an expert method for atoms, molecules, ions, valency, and chemical formulae.

These NCERT Solutions for Class 9 Science Chapter 9 are prepared by subject experts, matched to the 2026-27 NCERT Exploration textbook, and checked against the official NCERT answer key.

• 15 solved questions from the Revise, Reflect, Refine exercise, covering ionic and covalent bonds, valency, and chemical formulae.
• Every formula question shows the criss-cross rule, the working, and the final answer as separate steps, so nothing is skipped.
• Based on the new 2026-27 NCERT Exploration textbook, with an expert method added to each question.

Atoms and Molecules Class 9 One-Shot Video Explanation

Source: Physics Wallah on YouTube

This one-shot video walks through atoms, molecules, ions, valency, and formula writing before you start the exercise. Watch it once, then use the step-by-step solutions below to practise each question. The video and these solutions together cover the whole of Chapter 9 Atomic Foundations of Matter.

What Atomic Foundations of Matter Class 9 Science Covers

Chapter 9 is the chemistry heart of Class 9 Science. It explains how tiny atoms join to build every material around us. The chapter runs on a few clear ideas, and once you know them, most questions become quick to solve.

• Law of conservation of mass: in any chemical change, the total mass of the reactants equals the total mass of the products.
• Atoms, ions, and molecules: atoms gain, lose, or share electrons to become stable, forming cations, anions, and molecules.
• Bonding and formulae: ionic and covalent bonds, valency, the criss-cross rule, and the formula unit mass of a compound.

A stable atom has a full outer shell. Metals usually give away electrons and non-metals take or share them, and that single idea drives almost every answer in this chapter.

How Collegedunia's Class 9 Atomic Foundations NCERT Solutions Help You

This chapter mixes short reasoning questions with formula writing and simple sums. These solutions make each type easy to attempt and easy to score.

• 2026-27 NCERT alignment: Every answer matches the new Exploration textbook and the official NCERT answer key.
• Full working shown: The concept, the criss-cross or mass step, and the final answer sit on separate lines, so you never lose method marks.
• Expert method added: A second, faster approach is given below each solution to save time in the exam.
• Concepts made clear: Each solution starts with the exact idea it uses, such as valency, octet, or the law of conservation of mass.

Atomic Foundations of Matter Class 9 Important Formulas and Rules

This chapter is built on three rules rather than many formulas. Learn these first, and most exercise questions fall into place. Formula unit mass is measured in the unified mass unit (u), the same unit used for atomic mass.

The criss-cross rule is the single most useful tool here. Write each ion with its valency, swap the numbers to become subscripts, bracket any polyatomic ion taken more than once, and reduce to the simplest ratio. Always check that the total positive charge equals the total negative charge.

Ionic Bond and Covalent Bond in Class 9 Science

Atoms bond in two main ways, and telling them apart is key to answering the bonding questions in this chapter.

• Ionic bond: a metal gives electrons to a non-metal. This makes a positive cation and a negative anion that attract each other, as in sodium chloride.
• Covalent bond: two non-metals share electrons. This makes a neutral molecule, as in water or oxygen gas.
• Conductivity clue: ionic compounds conduct electricity when molten or dissolved, because their ions are then free to move. Solid ionic compounds and covalent compounds do not.

A quick test decides the bond type: metal plus non-metal usually means ionic, while non-metal plus non-metal means covalent. Questions on element A and B in the exercise use exactly this rule.

Revise, Reflect, Refine Exercise Breakdown for Class 9 Science Chapter 9

The Chapter 9 exercise is called Revise, Reflect, Refine and has 15 questions. The table shows what each question tests, so you can plan your practice.

Start with the reasoning questions, then move to formula writing and the mass sums. All 15 fully solved answers, with an expert method for each, are given lower down this page.

Atomic Foundations of Matter Weightage for the Class 9 Science Exam

Chapter 9 sits in the chemistry part of Class 9 Science. It carries steady marks every year, mostly through formula writing and short reasoning. The table compares it with the other chemistry chapters.

Because formula writing is rule-based, this chapter is one of the easiest places to score full marks with a little practice.

Common Mistakes Students Make in the Atomic Foundations Chapter

Solved Example: Writing a Formula by the Criss-Cross Rule

Write the formula of aluminium oxide, made of Al³⁺ and O^2- ions. This follows the same method used in Questions 5, 6, and 9 of the exercise.

1. Write the ions with valencies: aluminium is Al³⁺ (valency 3) and oxide is O^2- (valency 2).
2. Criss-cross the valencies: the 3 from aluminium becomes the subscript of oxygen, and the 2 from oxygen becomes the subscript of aluminium, giving Al₂O₃.
3. Reduce if needed: the subscripts 2 and 3 share no common factor, so the formula stays Al₂O₃.
4. Check neutrality: 2 × (+3) + 3 × (−2) = +6 − 6 = 0, so the compound is neutral and correct.

Every formula question in this chapter follows this same four-step pattern: write the ions, criss-cross, reduce, and check that the charges cancel.

How to Use the Class 9 Atomic Foundations Solutions Most Effectively

Use a simple three-step routine so the chapter stays in your memory until the exam.

• Learn the valencies first of the common ions and the three main rules of the chapter.
• Attempt each question yourself, then compare with the step-by-step solution below.
• Redo the formula questions you got wrong two days later, without looking at the answer.

Pair this page with the chapter notes and formula sheet for quick revision before the exam.

NCERT Class 9 Science Chapter 9 Other Study Resources

Use the other free resources for the same chapter alongside these solutions.

NCERT Solutions for Class 9 Science: All Chapters

Find the step-by-step NCERT Solutions for the other Class 9 Science chapters below.