Atoms and Molecules Notes – uppathshala

📘Class 9 Science – Chapter: Atoms and Molecules Notes

✨ Introduction

Matter is anything that has mass and occupies space.
All matter is composed of tiny units called atoms and molecules.
This chapter focuses on how atoms combine to form molecules and how mass is conserved in reactions.

🔹 Laws of Chemical Combination

1. Law of Conservation of Mass – Antoine Lavoisier

“Mass can neither be created nor destroyed in a chemical reaction.”
The total mass of reactants = Total mass of products.
Example:
- H₂ + Cl₂ → 2HCl
- Mass before = Mass after

2. Law of Constant Proportions – Joseph Proust

“In a pure substance, elements are always present in a definite proportion by mass.”
Example: In water (H₂O), the mass ratio of hydrogen to oxygen is always 1:8.

👁️‍🗨️ Atoms

The smallest particle of an element that retains its chemical properties.
Cannot be divided further by chemical means.

Characteristics of Atoms:

Represented by symbols (introduced by Dalton, modern system by Berzelius).
Unique for each element.
Indivisible in ordinary chemical reactions.

Element	Symbol	Atomic Mass (u)	Nature
Hydrogen	H	1 u	Non-metal
Carbon	C	12 u	Non-metal
Oxygen	O	16 u	Non-metal
Sodium	Na	23 u	Metal
Chlorine	Cl	35.5 u	Non-metal

⚖️ Atomic Mass

The relative mass of an atom compared to 1/12th the mass of carbon-12.
Unit: atomic mass unit (u).
Not necessarily a whole number due to isotopes.

🔹 Molecules

Formed when two or more atoms combine chemically.
Represents the smallest unit of a compound or elemental gas.

Types of Molecules:

Molecule of an Element: H₂, O₂, N₂
Molecule of a Compound: H₂O, CO₂, NH₃

Properties:

Can be diatomic (O₂), triatomic (O₃), or polyatomic (P₄, S₈).
Molecules retain chemical properties of the substance.

🔹 Atomicity

Number of atoms in one molecule of a substance.

Substance	Atomicity	Example Type
Helium (He)	1	Monoatomic
Oxygen (O₂)	2	Diatomic
Ozone (O₃)	3	Triatomic
Phosphorus (P₄)	4	Polyatomic

📘 Chemical Symbols

Proposed by Berzelius.
First letter capital, second letter (if present) lowercase.
Example: H for Hydrogen, He for Helium, Na for Sodium.

📋 Chemical Formulae

Shows elements in a compound and their ratios.
Formed using symbols and valencies.

Steps to Write a Formula:

Write the symbols.
Write the valencies.
Cross-multiply the valencies.
Simplify if needed.

Example:

Calcium chloride → Ca (valency 2), Cl (valency 1)
→ Formula: CaCl₂

Compound	Elements Used	Formula	Valency Cross Rule
Water	Hydrogen, Oxygen	H₂O	H(1), O(2) → H₂O
Ammonia	Nitrogen, Hydrogen	NH₃	N(3), H(1) → NH₃
Magnesium oxide	Mg, O	MgO	Mg(2), O(2) → MgO

⚖️ Molecular Mass

Sum of atomic masses of all atoms in a molecule.
Unit: unified atomic mass (u)

Formula:

$Molecular Mass=∑(Atomic mass×Number of atoms)\text{Molecular Mass} = \sum (\text{Atomic mass} \times \text{Number of atoms})$

Examples:

H₂O = (2 × 1) + (1 × 16) = 18 u
CO₂ = (1 × 12) + (2 × 16) = 44 u

📦 Formula Unit Mass

Applies to ionic compounds.
Sum of atomic masses of ions in a formula unit.

Examples:

NaCl = 23 + 35.5 = 58.5 u
CaCl₂ = 40 + (2 × 35.5) = 111 u

🔢 Mole Concept

Introduced by Avogadro.
1 mole = 6.022 × 10²³ particles (Avogadro’s number)
Molar mass = Mass of 1 mole of a substance in grams

Key Formulas:

Number of moles = Mass (g) / Molar mass (g/mol)
Number of particles = Moles × 6.022 × 10²³
Molar volume (for gases at STP) = 22.4 L/mol

Example Calculations:

Mass of 2 moles of H₂O:
= 2 × 18 g = 36 g
Particles in 0.5 moles of NaCl:
= 0.5 × 6.022 × 10²³ = 3.011 × 10²³ particles

📍 Important Definitions

Atom: Smallest particle of an element.
Molecule: Smallest particle of a compound or element that can exist independently.
Ion: An atom or group of atoms with a charge.
Molecular Mass: Mass of one molecule.
Formula Unit Mass: Mass of one formula unit of an ionic compound.
Mole: SI unit for the amount of substance.