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Notes of Ch 3 Atoms and Molecules| Class 9th Science

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Study Material and Notes of Ch 3 Atoms and Molecules Class 9th Science

Topics in the Chapter

• Introduction
• Laws of Chemical Combination
→ Law of Conservation of Mass
→ Law of Constant Proportions
• Dalton's Atomic Theory
• Atoms
→ Modern day symbols of Elements
→ Atomic Mass
→ Atom existence
• Molecules
→ Molecules of Elements
→ Molecules of Compounds
→ What is an ion?
• Writing a Chemical Formulae
→ Formulae of simple compounds
• Molecular Mass
→ Formula Unit Mass
• Mole Concept

Introduction

→ Around 500 B.C., Indian philosopher Maharishi Kanad, postulated the theory if we go on dividing matter (padarth), we will obtain smallest particle beyond which further division can't be possible which is known as 'parmanu'.

→ Ancient Greek philosophers – Democritus and Leucippus called these particles atoms.

→ Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Laws of Chemical Combination

• This law established after the experiments by Lavoisier and Joseph L. Proust.

• The chemical reaction between two or more substances give rise to products which is governed by certain laws called Laws of Chemical Combination.
• Law of Conservation of Mass

→ During a chemical reaction, the total mass of reactants will be equal to the total mass of the products.

→ Mass can neither be created nor destroyed in a chemical reaction.

→ Example: A (reactant) + B (reactant) → AB (product)
mass of A + mass of B = mass of AB

• Law of Constant Proportions

→ In a chemical reaction, compounds always contain the same elements present in definite proportions by mass irrespective of their source.

→ It was given by Lavoisier.

→ For example: (i) 18 gm of H2O = 2 gm of hydrogen + 16 gm of oxygen
⇒ mass of hydrogen/mass of oxygen = 2/16 = 1/8
(ii) 36 gm of H2O = 4 gm of hydrogen + 32 gm of oxygen
⇒ mass of hydrogen/mass of oxygen = 4/32 = 1/8
(iii) 9 gm of H2O = 1 gm of hydrogen + 8 gm of oxygen
⇒ mass of hydrogen/mass of oxygen = 1/8
This verifies law of constant proportions as the ratio of mass of hydrogen to oxygen is always same.

Dalton's Atomic Theory

→ According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Six Postulates of Dalton's atomic theory:

(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties. (Law of conservation of mass)
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds. (Law of constant proportion)
(vi) The relative number and kinds of atoms are constant in a given compound.

Atoms

• Atoms are building blocks of all matter.

→ Modern day symbols of Elements
→ Atomic Mass
→ Atom existence

NCERT Solutions of Atoms and Molecules

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