ATOMS AND MOLECULES

 CHAPTER 3

CLASS – IX

ATOMS AND MOLECULES

1. Laws of Chemical Combination

These two fundamental laws govern how elements combine to form compounds.

• Law of Conservation of Mass (Lavoisier):
• Statement: Mass can be neither created nor destroyed in a chemical reaction.
• Application: The total mass of reactants must be exactly equal to the total mass of products. This law explains why we balance chemical equations.
• Law of Constant Proportions (Proust):
• Statement: A pure chemical compound always contains the same elements combined in the same fixed ratio by mass, regardless of its origin.
• Example: CO₂​ always has Carbon and Oxygen in a mass ratio of 3:8.

 

2. Dalton's Atomic Theory

John Dalton's theory (1808) was the first scientific explanation for the laws above.

• Key Postulates:

1.    All matter is composed of tiny, ultimate particles called atoms.

2.    Atoms are indivisible.

3.    Atoms of the same element are identical in mass and properties.

4.    Atoms combine in the ratio of small whole numbers to form compounds.

5.    Atoms are neither created nor destroyed in a chemical reaction.

• Modern Modifications: Postulates 2 and 3 are no longer strictly true. Atoms are divisible (into subatomic particles), and atoms of the same element can have different masses (isotopes).

Term	Definition	Key Characteristics & Examples
Atom	The smallest particle of an element. May or may not exist independently.	Represented by symbols (H, Fe). Most are highly reactive.
Molecule	An independent, stable group of two or more atoms chemically bonded together.	Elemental: O2, P4 (Same atoms). Compound: H2O, CO2​ (Different atoms).
Atomicity	The number of atoms constituting one molecule.	Diatomic (O2​), Tetra-atomic (P4), Polyatomic (S8​).
Ion	A charged particle is formed by the gain or loss of electrons.	Cation (Na+, loses e−). Anion (Cl−, gains e−).
Polyatomic Ion	A group of atoms carrying a net charge that behaves as a single unit.	Sulphate (SO4 2-), Carbonate (CO3 2-).

4. Writing Chemical Formulae

A chemical formula is a symbolic representation of a compound's composition.

• Valency (Combining Capacity): The power of an element to combine with others. It is numerically equal to the charge on the ion.
• The Criss-Cross Method:

1.    Write the symbols of the element/ion and its valency (or charge).

2.    Criss-cross the valencies to become the subscripts for the other atom/ion.

3.    Example: Calcium Chloride (Ca²⁺ and Cl^1-). Criss-cross gives CaCl₂

• Rules for Ionic Compounds:

• The charges must be balanced (the net charge must be zero).
• Metals (cations) are written first, then non-metals (anions).
• Use parentheses for polyatomic ions only when their number is two or more (e.g., Mg(OH)₂
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5. Atomic and Formula Mass (Calculations)

• Atomic Mass Unit (u): The standard unit for mass used at the atomic level, defined as 1/12 th the mass of a Carbon-12 atom.
• Atomic Mass: The average mass of an atom of an element relative to the 1/12​th mass of a Carbon-12 atom.
• Formula Mass (or Molecular Mass):
• Definition: The sum of the atomic masses of all atoms present in a compound's formula unit.
• Calculation: Sum (Number of Atoms × Atomic Mass) for every element.
• Example: H₂SO₄ (Atomic masses: H=1u,S=32u,O=16u)

Formula Mass= (2×1u)+(1×32u)+(4×16u)=98u