Chapter 9: Atomic Foundations of Matter
Original notes — ready CBSE (NCERT, English Medium) • Class 9 • Science • Book: Exploration
Chapter 9: Atomic Foundations of Matter — EG's own notes, in simple words
Book: Exploration (CBSE, Class 9) | Chapter 9 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): A water molecule: one oxygen atom joined to two hydrogen atoms. A general diagram for study, not a figure copied from the book.
Atoms joining to form molecules. A general picture about the building blocks of matter, not a book scene.
What the chapter is about
Everything around us is made of atoms, but atoms rarely stay alone. They join together to form molecules, the smallest units of a substance that keep its properties. A molecule of water has two hydrogen atoms and one oxygen atom, written H₂O, while a molecule of oxygen gas has two oxygen atoms, written O₂.
Atoms join by sharing or transferring electrons, and this decides the kind of bond. In a covalent bond, atoms share electrons, as in water. In an ionic bond, one atom gives electrons to another, forming charged particles called ions — a positive cation and a negative anion — which attract each other, as in common salt, sodium chloride.
Chemists compare atoms using atomic mass, measured against a standard, and molecules using molecular mass, the sum of the atomic masses in the molecule. Atoms also have valency, their combining capacity, which decides the formula of a compound: the valencies of the elements are crossed over to write the correct formula.
What you should learn
Notice how atoms combine into molecules, how ionic and covalent bonds differ, and how valency decides a chemical formula.
Key Terms
- Atom — The smallest unit of an element that takes part in a chemical reaction.
- Molecule — Two or more atoms joined together, forming the smallest unit of a substance.
- Ion — An atom or group of atoms with a charge, formed by gaining or losing electrons.
- Valency — The combining capacity of an atom — how many electrons it can share, gain or lose.
- Atomic mass — The mass of an atom compared with a standard atom, given in atomic mass units.
Remember
- A molecule keeps the properties of its substance.
- Covalent bond = sharing electrons; ionic bond = transfer of electrons.
- Ions: cations are positive, anions are negative.
- Formulae are written by crossing over valencies.
Exam Tip / Common Mistake
Common mistake: writing the formula of water as HO₂ or H₂O₂. Water is H₂O — two hydrogen atoms for every oxygen atom. Always check the valency of each element before writing a formula.
Chart / Table: Writing a formula from valencies
| Point | Detail |
|---|---|
| Water | H (valency 1) + O (valency 2) → H₂O |
| Common salt | Na (valency 1) + Cl (valency 1) → NaCl |
| Carbon dioxide | C (valency 4) + O (valency 2) → CO₂ |
| Ammonia | N (valency 3) + H (valency 1) → NH₃ |
Practice set — try it yourself
- What is a molecule? How is it different from an atom?
Hint: Think about atoms joining together. - Write the formula of water and explain it using valency.
Hint: Think about hydrogen valency 1 and oxygen valency 2. - How is an ionic bond different from a covalent bond?
Hint: Think about transfer versus sharing. - What is an ion? What is a cation?
Hint: Think about gaining and losing electrons. - Calculate the molecular mass of water (H = 1, O = 16).
Hint: Think about adding the atomic masses.
Questions with Answers
- What is a molecule?
A molecule is two or more atoms joined together by chemical bonds. It is the smallest unit of a substance that keeps the properties of that substance. - How do ionic and covalent bonds differ?
In an ionic bond, electrons are transferred from one atom to another, forming oppositely charged ions that attract each other. In a covalent bond, atoms share electrons. - What is valency, and how does it help write formulae?
Valency is the combining capacity of an atom. To write a formula, the valencies of the elements are crossed over: the valency of one element becomes the subscript of the other, giving the correct ratio of atoms. - Why is the formula of water H₂O?
Hydrogen has valency 1 and oxygen has valency 2. Crossing over the valencies gives two hydrogen atoms for every oxygen atom, so the formula is H₂O. - What is molecular mass? Calculate it for carbon dioxide (C = 12, O = 16).
Molecular mass is the sum of the atomic masses of all atoms in a molecule. For CO₂ it is 12 + (2 × 16) = 44 u.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.