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Grade 9/ Science/ Atoms and Molecules
Chapter 3 · NCERT Class 9 Science

Atoms and Molecules

Everything around you — water, air, your own body — is built from a small set of atoms joined in fixed ways. Learn the rules they follow, the names we give them, and how to weigh a molecule. Tap each term to begin.

⚛️ 3 topics⏱ ~25 min📝 12-question quiz
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The building blocks of matter

Atom, molecule, element, compound, atomicity, ion — six words that come up again and again. Tap each to see exactly what it means and an everyday example.

Explore · The vocabulary of chemistrytap a term

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The three big ideas

  • Law of conservation of mass — in a chemical reaction, mass is neither created nor destroyed. The total mass of the reactants equals the total mass of the products.
  • Law of constant proportions — a pure compound always contains the same elements combined in the same fixed ratio by mass. Water, for example, is always hydrogen and oxygen in a 1:8 mass ratio, no matter the source.
  • Together these laws led to the idea that matter is made of tiny, indivisible particles — atoms.
  • An atom is the smallest particle of an element. An element is made of only one kind of atom (oxygen, gold); a compound joins two or more elements in a fixed ratio (water, H₂O).
  • A molecule is two or more atoms chemically bonded — of an element (O₂) or of a compound (H₂O, CO₂).
  • Symbols & formulae: every element has a short symbol (H, O, Na, Fe). A chemical formula shows which elements are present and how many atoms — H₂O means 2 hydrogen atoms + 1 oxygen atom.
Common mistake: thinking only compounds have molecules. A molecule of an element (like O₂) is made of identical atoms; a molecule of a compound (like H₂O) has different elements — but both are molecules.
  • The molecular mass of a substance is the sum of the atomic masses of all the atoms in its formula. It is measured in atomic mass units (u).
  • Just count each kind of atom in the formula, multiply by its atomic mass, and add.

Worked example. Find the molecular mass of water, H₂O (H = 1 u, O = 16 u).

Water has 2 hydrogen atoms and 1 oxygen atom, so molecular mass = (2 × 1) + 16 = 2 + 16 = 18 u.

Where you'll meet it

Atoms and molecules, in real life

Medicine & manufacturing

Chemical formulae and molecular masses let chemists and pharmacists measure out the exact amounts of each substance — too little or too much medicine can be useless or dangerous.

Cooking & digestion

Baking, fermenting and even digesting your food are chemical combinations — atoms rearranging into new molecules, always obeying conservation of mass.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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Interactive built to the OpenMAIC approach (THU-MAIC, MIT). Content from the NCERT Class 9 Science textbook (ncert.nic.in).

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