CHAPTER 3: ATOMS AND MOLECULES
🔹 Introduction
All matter is made up of small particles called
atoms and molecules.
This chapter explains laws of chemical combination and
chemical calculations.
🔹 Laws of Chemical Combination (Very Important)
1️⃣Law of Conservation of Mass
Statement:
Mass can neither be created nor destroyed in a
chemical reaction.
Scientist: Antoine Lavoisier
Year: 1774
Example:
CaCO3 →CaO +CO2
Total mass of reactants = Total mass of products
2️⃣Law of Constant Proportions
Statement:
A pure chemical compound always contains the same
elements combined in a fixed proportion by mass.
Scientist: Joseph Proust
Year: 1799
Example:
Water (H₂O) always has H : O = 1 : 8 by mass.
🔹 Dalton’s Atomic Theory
Scientist: John Dalton
Year: 1808
Dalton’s atomic theory
, ● All matter is made of very tiny particles called
atoms, which participate in chemical reactions.
● Atoms are indivisible particles, which cannot be
created or destroyed in a chemical reaction.
● Atoms of a given element are identical in mass
and chemical properties.
● Atoms of different elements have different masses
and chemical properties.
● Atoms combine in the ratio of small whole numbers
to form compounds.
● The relative number and kinds of atoms are
constant in a given compound.
🔹 What is an Atom?
An atom is the smallest particle of an element that
takes part in a chemical reaction.
Atoms are very small and cannot exist freely (except
noble gases).
🔹 What is a Molecule?
A molecule is the smallest particle of a substance
that can exist independently.
Types of Molecules:
1. Molecules of elements – O₂, N₂
2. Molecules of compounds – H₂O, CO₂
🔹 Atomicity
Atomicity = Number of atoms present in one molecule
of an element.
Element Atomicity
Helium 1
(He) (Monoatomic)
Oxygen (O₂) 2
🔹 Introduction
All matter is made up of small particles called
atoms and molecules.
This chapter explains laws of chemical combination and
chemical calculations.
🔹 Laws of Chemical Combination (Very Important)
1️⃣Law of Conservation of Mass
Statement:
Mass can neither be created nor destroyed in a
chemical reaction.
Scientist: Antoine Lavoisier
Year: 1774
Example:
CaCO3 →CaO +CO2
Total mass of reactants = Total mass of products
2️⃣Law of Constant Proportions
Statement:
A pure chemical compound always contains the same
elements combined in a fixed proportion by mass.
Scientist: Joseph Proust
Year: 1799
Example:
Water (H₂O) always has H : O = 1 : 8 by mass.
🔹 Dalton’s Atomic Theory
Scientist: John Dalton
Year: 1808
Dalton’s atomic theory
, ● All matter is made of very tiny particles called
atoms, which participate in chemical reactions.
● Atoms are indivisible particles, which cannot be
created or destroyed in a chemical reaction.
● Atoms of a given element are identical in mass
and chemical properties.
● Atoms of different elements have different masses
and chemical properties.
● Atoms combine in the ratio of small whole numbers
to form compounds.
● The relative number and kinds of atoms are
constant in a given compound.
🔹 What is an Atom?
An atom is the smallest particle of an element that
takes part in a chemical reaction.
Atoms are very small and cannot exist freely (except
noble gases).
🔹 What is a Molecule?
A molecule is the smallest particle of a substance
that can exist independently.
Types of Molecules:
1. Molecules of elements – O₂, N₂
2. Molecules of compounds – H₂O, CO₂
🔹 Atomicity
Atomicity = Number of atoms present in one molecule
of an element.
Element Atomicity
Helium 1
(He) (Monoatomic)
Oxygen (O₂) 2
