This is your complete guide to Atoms AND Molecules for Class 9th Science. Read clear, exam-focused notes, understand the key concepts step by step, and then test yourself with multiple-choice questions, a quick quiz and printable worksheets. Atoms AND Molecules is an important part of the Class 9th Science syllabus and regularly appears in exams, so mastering it now will boost both your understanding and your marks. Everything here is free, organised in one place, and built to make your revision faster and more effective.
Atoms AND Molecules is a key chapter in the Class 9th Science syllabus. In this chapter you'll build a clear understanding of its core ideas, learn how they connect to one another, and see how they apply to questions you'll face in your exams. The goal is not just to memorise, but to understand — so you can solve any question, however it's framed.
Along the way you'll come across important definitions, concepts and problem-solving methods that form the foundation for later chapters and higher classes. Many real-life situations and other topics rely on the ideas introduced here, which is exactly why examiners give this chapter steady weightage.
Use the notes below to learn the concepts, the MCQs and quiz to test yourself, and the worksheet and important questions to revise thoroughly before your exam.
Atoms AND Molecules carries steady weightage in Class 9th exams. Practising its MCQs and important questions is one of the fastest ways to secure marks from this chapter.
Clear, step-by-step notes to help you understand every concept in this chapter.
Everything around us is made up of tiny particles called atoms and molecules.
An atom is the smallest particle of an element that cannot normally exist independently and retains all the chemical properties of that element. Different elements have different types of atoms. The idea of atoms was first given by John Dalton in his Atomic Theory. He said that matter is made up of small particles called atoms, which are indivisible and indestructible. Atoms of the same element are identical in mass and properties, and they combine in fixed ratios to form compounds.
A molecule is a group of two or more atoms that are chemically bonded together. Molecules can be of elements or compounds. Molecules of elements contain the same type of atoms, such as O₂, N₂, and H₂. Molecules of compounds contain different types of atoms, such as H₂O, CO₂, and NH₃.
The chemical formula of a substance shows the composition of its molecules using symbols and numbers. For example, H₂O represents two atoms of hydrogen and one atom of oxygen.
Atoms or groups of atoms that carry a charge are called ions. Positively charged ions are called cations, like Na⁺ and Mg²⁺, while negatively charged ions are called anions, like Cl⁻ and SO₄²⁻.
There are two important laws of chemical combination:
The mole concept is used to count particles such as atoms, molecules, or ions. One mole contains 6.022 × 10²³ particles, known as Avogadro’s number. The molar mass of a substance is the mass of one mole of that substance in grams.
In short, atoms are the building blocks of matter, and molecules are formed when atoms combine. Understanding atoms and molecules helps us know how substances combine and react to form new materials.
Quick, must-remember points and formulas from Atoms AND Molecules.
Get all the important Science formulas in one quick-revision sheet.
Open Formula SheetsAttempt these multiple-choice questions, then reveal the answer to check yourself.
(a) Quick lime (CaO) ----> Calcium and Oxygen
(b) Hydrogen bromide (HBr) ----> Hydrogen and Bromine
(c) Baking powder (NaHCO₃) -----> Sodium, Hydrogen, Carbon, and Oxygen
(d) Potassium sulphate (K₂SO₄) ------> Potassium, Sulphur, and Oxygen
Polyatomic ions are ions that contain two or more atoms joined together by covalent bonds and carry a net positive or negative charge. These atoms act as a single charged unit during chemical reactions.
Examples:
Thus, polyatomic ions are groups of atoms that behave as one charged particle.
The scientist who laid the foundation of chemical sciences was Antoine Lavoisier.
He is known as the Father of Modern Chemistry because he:
Scientists chose 1/16 of the mass of an atom of naturally occurring oxygen as the atomic mass unit because oxygen reacts with many elements to form compounds and is abundantly available in nature, making it a convenient and reliable standard for comparing atomic masses of different elements.
There are 6.022 × 10²³ atoms in exactly 12 g of carbon.
This number is known as the Avogadro number, and 12 g of carbon represents 1 mole of carbon atoms.
The two atoms which can exist independently are Helium (He) and Argon (Ar).
These are noble gases and exist as single atoms because they have a completely filled outermost electron shell, making them stable and unreactive.
The atomicity of Argon is 1.
Argon is a noble gas and exists as a single atom (monatomic) because it has a completely filled outermost electron shell, making it stable and unreactive.
In one molecule of H₂SO₄ (sulphuric acid):
Hydrogen (H) = 2 atoms
Sulphur (S) = 1 atom
Oxygen (O) = 4 atoms
Total atoms = 2 + 1 + 4 = 7 atoms
There are 7 atoms in one molecule of H₂SO₄.
(a) Magnesium chloride ----> MgCl₂
(b) Calcium oxide ----> CaO
(c) Copper nitrate -----> Cu(NO₃)₂
(d) Aluminium chloride ----> AlCl₃
(e) Calcium carbonate ------> CaCO₃
The Latin name of sodium is Natrium.
The Latin name of mercury is Hydrargyrum.
The Law of Constant Proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, regardless of its source or method of preparation.
Example:
Water (H₂O) always contains hydrogen and oxygen in the ratio of 1:8 by mass, whether it is obtained from a river, rain, or produced in a laboratory.
Molecular mass is the sum of the atomic masses of all the atoms present in a molecule. It represents the total mass of one molecule of a substance and is expressed in atomic mass units (amu).
Example:
The molecular mass of water (H₂O) = (2 × 1) + 16 = 18 amu.
The atomic number of magnesium (Mg) is 12.
In a neutral Mg atom, the number of electrons = number of protons = 12.
In a Mg²⁺ ion, the atom loses 2 electrons, so the number of electrons = 12 − 2 = 10.
Therefore:
Mg atom → 12 electrons
Mg²⁺ ion → 10 electrons
The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.
According to this law, the total mass of the reactants is always equal to the total mass of the products.
Example:
When hydrogen reacts with oxygen to form water,
2H₂ + O₂ → 2H₂O
The total mass of hydrogen and oxygen before the reaction is equal to the mass of water formed after the reaction.
The element used as the reference for atomic mass is Carbon.
The isotope Carbon-12 (C-12) is taken as the standard reference, and its atomic mass is defined as exactly 12 atomic mass units (amu)
Oxygen (O₂) is a diatomic gas.
From the data:
3.0 g of carbon + 8.0 g of oxygen ----> 11.0 g of carbon dioxide
This means 3 g of carbon requires 8 g of oxygen to form 11 g of carbon dioxide.
Now, if 3 g of carbon is burnt in 50 g of oxygen, carbon is the limiting reactant because it can combine only with 8 g of oxygen. The remaining oxygen will be left unused.
So, the amount of carbon dioxide formed will still be 11 g.
Mass of carbon dioxide formed = 11.0 g
Law governing the reaction:
This is governed by the Law of Constant Proportion, which states that a chemical compound always contains the same elements in the same fixed ratio by mass.
The organisation that approves the names of elements all over the world is the International Union of Pure and Applied Chemistry (IUPAC).
The symbol of gold is Au
To calculate the formula unit mass of calcium chloride (CaCl₂):
Atomic mass of Ca = 40
Atomic mass of Cl = 35.5
Formula unit mass = 40 + (2 × 35.5) = 40 + 71 = 111 u
So Formula unit mass of CaCl₂ is 111 u
Total mass of the compound = 0.24 g
Mass of boron = 0.096 g
Mass of oxygen = 0.144 g
Percentage of boron = (0.096 / 0.24) × 100 = 40%
Percentage of oxygen = (0.144 / 0.24) × 100 = 60%
Therefore, the percentage composition of the compound is:
Boron = 40%
Oxygen = 60%
Polyatomic ions are ions that consist of two or more atoms bonded together covalently and carry a net positive or negative charge. These atoms act as a single charged unit in chemical reactions.
Examples:
The law of conservation of mass was given by Antoine Lavoisier in 1774.
He stated that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants is always equal to the total mass of the products.
Lavoisier proved this law through careful experiments, showing that even when substances change form during a reaction, their total mass remains the same.
(a) Ethyne (C₂H₂)
= (2 × 12) + (2 × 1)
= 24 + 2
= 26 g/mol
(b) Sulphur molecule (S₈)
= 8 × 32
= 256 g/mol
(c) Phosphorus molecule (P₄)
= 4 × 31
= 124 g/mol
(d) Hydrochloric acid (HCl)
= (1 × 1) + (1 × 35.5)
= 1 + 35.5
= 36.5 g/mol
(e) Nitric acid (HNO₃)
= (1 × 1) + (1 × 14) + (3 × 16)
= 1 + 14 + 48
= 63 g/mol
The term used to represent the mass of 1 mole molecules of a substance is molar mass. It is expressed in grams per mole (g/mol).
The number 6.023 × 10²³ is called the Avogadro number or Avogadro constant. It represents the number of particles (atoms, molecules, or ions) present in one mole of a substance.
A short, timed quiz with instant scoring — perfect for checking how well you know the chapter.
Attempt a 10–20 question quiz on Atoms AND Molecules. Try to finish within 15 minutes, get instant scoring, and see which topics need more revision.
Start Quiz NowPractise every question type — from one-mark fill-in-the-blanks to long-answer questions.
Quick 1–3 mark questions to test core concepts.
Descriptive questions to build complete, exam-ready answers.
One-word and fill-up questions for quick revision.
Commonly asked questions from past exams.
High-weightage and frequently asked questions to prioritise before exams.
A one-page recap to revise the whole chapter in minutes.
Educational intent: This page is created to help Class 9th students learn and revise Atoms AND Molecules from Science using notes, practice questions and free study tools.
Accuracy & learning-first: Our content is prepared and reviewed by experienced educators and kept aligned with the latest NCERT / CBSE syllabus and exam pattern.
Student-focused note: These resources support your school learning and self-study. Always cross-check with your prescribed textbook and your teacher's guidance for board examinations.