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Atoms and Molecules

CBSE Class 9 Science • Chapter 3 • Detailed Master Notes

Chapter Overview:

This chapter lays the foundation of Chemistry. It introduces the Laws of Chemical Combination, Dalton's Atomic Theory, the concept of Atoms, Atomic Mass, Molecules, writing Chemical Formulas, and the very important Mole Concept.

1. Laws of Chemical Combination

(i) Law of Conservation of Mass (Lavoisier):

Mass can neither be created nor destroyed in a chemical reaction. Total mass of reactants = Total mass of products.

(ii) Law of Constant Proportions (Proust):

In a pure chemical substance, elements are always present in definite proportions by mass.
e.g., In water ($H_2O$), ratio of mass of Hydrogen to Oxygen is always $1:8$, whatever the source.

2. Dalton's Atomic Theory

All matter is made of very tiny particles called atoms.
Atoms are indivisible particles involving chemical reactions.
Atoms of a given element are identical in mass and properties.
Atoms of different elements have different masses and properties.
Atoms combine in ratio of small whole numbers to form compounds.

3. What is an Atom?

The smallest particle of an element that retains its chemical properties.

Reactivity: Atoms are very reactive and generally not found in free state (except Noble gases).
Size: Atomic radius is measured in nanometers. $1 nm = 10^{-9} m$.
Atomic Mass Unit (u): Mass exactly equal to $1/12^{th}$ the mass of one carbon-12 atom.

4. Molecules and Ions

Molecule: A group of two or more atoms chemically bonded. It can exist independently.

Molecules of Elements: Same type of atoms. e.g., $O_2$ (Diatomic), $P_4$ (Tetra-atomic).
Molecules of Compounds: Different atoms. e.g., $H_2O$, $NH_3$.

Ion: A charged species.

Cation: Positive charge ($Na^+$).
Anion: Negative charge ($Cl^-$).
Polyatomic Ion: Group of atoms carrying charge (e.g., Ammonium $NH_4^+$, Sulphate $SO_4^{2-}$).

5. Writing Chemical Formulas

Formulae are written by crossing valencies.

Example: Magnesium Chloride

Symbol: Mg, Cl
Valency: +2, -1
Cross: $Mg_1 Cl_2 \Rightarrow MgCl_2$

Example: Aluminium Oxide

Symbol: Al, O
Valency: +3, -2
Cross: $Al_2 O_3 \Rightarrow Al_2O_3$

6. Molecular Mass and Mole Concept

This is the most critical calculation part of chemistry.

(a) Molecular Mass

Sum of atomic masses of all atoms in a molecule.

e.g., $H_2O = 2 \times 1 + 16 = 18 u$.

(b) Mole Concept

A mole is a number unit (like a dozen = 12). It links mass and number of particles.

1 Mole = $6.022 \times 10^{23}$ particles (Avogadro No., $N_0$).

1 Mole atoms = Gram Atomic Mass of element.

e.g., 1 Mole Carbon atoms = $12g$ Carbon = $6.022 \times 10^{23}$ atoms.

Number of Moles ($n$):

$$ n = \frac{\text{Given Mass (m)}}{\text{Molar Mass (M)}} $$

$$ n = \frac{\text{Given Number (N)}}{\text{Avogadro Number (N}_0)} $$

Practice Zone

Q1: Write formula of Calcium Carbonate.

Ans: Ca (+2), $CO_3$ (-2). Cross valencies: $CaCO_3$.

Q2: Convert 22g of Carbon Dioxide ($CO_2$) into moles.

Ans: Molar mass of $CO_2 = 12 + 2\times16 = 44g$.
Moles $n = m/M = 22/44 = 0.5$ moles.