Introduction
Welcome to the free notes for Class 9 Science Chapter 9 notes – Atomic Foundations of Matter. These notes are based on the latest NCERT Exploration: Textbook of Science for Grade 9. This chapter explains the basic concepts of atoms, molecules, elements, compounds, valency, chemical symbols, and chemical formulae. These notes are designed for quick revision and exam preparation.
Based on the Latest NCERT Textbook
These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and cover all important concepts in an easy-to-understand format.
Table of Contents
Laws of Chemical Combination
Matter combines according to certain fixed laws.
Law of Conservation of Mass
Mass can neither be created nor destroyed during a chemical reaction.
Law of Constant Proportions
A compound always contains the same elements in the same proportion by mass.
Dalton’s Atomic Theory
According to Dalton:
- Matter is made up of tiny particles called atoms.
- Atoms cannot be created or destroyed during chemical reactions.
- Atoms of the same element are identical.
- Atoms combine in simple whole-number ratios to form compounds.
What is an Atom?
An atom is the smallest particle of an element that retains its chemical properties.
Examples
- Hydrogen atom
- Oxygen atom
- Carbon atom
What is a Molecule?
A molecule is formed when two or more atoms combine chemically.
Examples
- H₂
- O₂
- H₂O
- CO₂
Chemical Symbols
Every element has a unique chemical symbol.
Examples
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Oxygen | O |
| Carbon | C |
| Nitrogen | N |
| Sodium | Na |
| Potassium | K |
| Calcium | Ca |
Atomic Mass
Atomic mass is the average mass of an atom expressed in atomic mass units (u).
Example:
- Carbon = 12 u
- Oxygen = 16 u
Molecular Mass
Molecular mass is the sum of the atomic masses of all atoms present in a molecule.
Example:
H₂O
= 2 × 1 + 16
= 18 u
Mole Concept
A mole is the amount of substance containing 6.022 × 10²³ particles.
This number is called Avogadro’s Number.
Valency
Valency is the combining capacity of an atom.
Examples
| Element | Valency |
|---|---|
| Hydrogen | 1 |
| Oxygen | 2 |
| Nitrogen | 3 |
| Carbon | 4 |
| Sodium | 1 |
| Magnesium | 2 |
| Aluminium | 3 |
Writing Chemical Formulae
Chemical formulae are written by balancing the valencies of the combining elements.
Examples
- Water → H₂O
- Carbon dioxide → CO₂
- Sodium chloride → NaCl
- Calcium chloride → CaCl₂
- Magnesium oxide → MgO
Difference Between Atom and Molecule
| Atom | Molecule |
|---|---|
| Smallest particle of an element | Combination of two or more atoms |
| Cannot be divided chemically | May contain atoms of same or different elements |
| Example: H | Example: H₂O |
Important Terms
- Atom
- Molecule
- Element
- Compound
- Chemical Symbol
- Atomic Mass
- Molecular Mass
- Mole
- Avogadro Number
- Valency
- Chemical Formula
Chapter Summary
✅ Matter is made of atoms.
✅ Molecules are formed by the chemical combination of atoms.
✅ Chemical symbols represent elements.
✅ Atomic mass is expressed in atomic mass units (u).
✅ Valency determines how atoms combine.
✅ Chemical formulae show the composition of compounds.
Complete Your Preparation
The notes above cover only the basic concepts of this chapter.
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FAQ
What is an atom?
An atom is the smallest particle of an element that retains its chemical properties.
What is a molecule?
A molecule is formed when two or more atoms chemically combine.
What is valency?
Valency is the combining capacity of an atom that determines how it forms chemical bonds.
What is Avogadro’s number?
Avogadro’s number is 6.022 × 10²³, which represents the number of particles in one mole of a substance.
Are these notes based on the latest NCERT book?
Yes. These notes are prepared according to the latest NCERT Exploration: Textbook of Science for Grade 9 and are suitable for CBSE students following the current syllabus.
Are these notes enough for exams?
These notes are ideal for quick revision. For complete preparation with detailed notes, NCERT solutions, mind maps, MCQs, assertion-reason questions, case-based questions, and practice papers, the complete eBook is recommended.
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