Atoms and Molecules Notes

Atoms and Molecules Class 9 notes – Chapter 3

Laws of chemical combination

Whenever reactants react together to form the products or the elements combine together to form a compound, they do this according to certain laws. These laws are called laws of chemical combination.

Law of conservation of mass

It states, “Mass can neither be created nor be destroyed during a chemical reaction.” This means that in any chemical reaction, the total mass of the reactants is equal to the total mass of the products and there is no change in mass during the chemical reaction. It is also known as law of indestructibility of matter.

Law of constant proportions / Law of definite proportions

“In a chemical substance (or compound), the elements are always present in definite proportions (or ratios) by mass.”

Explanation of laws if chemical combination :

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.


Atoms are the smallest particles of an element which may have or may not have independent existence but take part in a chemical reaction.

Atomic mass

According to Dalton’s, each element has a characteristic atomic mass. But determining the mass of an individual atom was a relatively difficult task due to its very small size.

Hence, their relative atomic masses were determined using the laws of chemical combination and the compounds formed. For this purpose, initially 1/16 of the mass of an atom of naturally occurring oxygen was taken as standard unit because of the following two reasons:

(i) Oxygen reacted with a large number of elements and formed compounds.

(ii) This unit gave masses of most of the elements as whole numbers.

However in 1961, carbon (C-12 isotope) was chosen as standard reference for measuring atomic masses universally.


The smallest particle of an element or compound which is capable of independent existence and shows all the properties of that substance is called a molecule.


It is defined as the number of atoms present in a molecule. On the basis of atomicity, molecules can be classified as:

(i) Monoatomic molecules: They consist of only one atom. e.g. He, Ne, Ar etc.

(ii) Diatomic molecules:  They consists of two atoms. e.g. H2,O2, N2 etc.

(iii) Triatomic molecules: They consists of three atoms. e.g. O3, CO2 etc.

(iv) Tetraatomic molecules: They consists of four atoms. e.g. P4,H2O2 etc.

(v) Polyatomic molecules: They consist of more than four atoms. e.g. CH4 (Penta-atomic), S8 (Octa-atomic) etc.


When atoms, groups of atoms or molecules lose or gain electron (s) they become charged. These charged species are known as ions.


They positively charged ions are known as cations.

e.g. Na+, K+ etc.


The negatively charged ions are known as anions.

e.g. Cl, Br etc.

Polyatomic ion

A group of atoms carrying charge a single entity is known as a polyatomic ion.

e.g. NO3


The combining power (or capacity) of an element is called its valency.

Molecular mass

Calculate the molecular mass of the following substances.

(i) Ammonia

 Sol: Molecular mass of ammonia (NH3

= 1 X 14 + 3 X 1 = 17 u

Try To Solve These Questions

Q1. What is 1 amu?

Q2. What is Law of definite proportions.

Q3. Define Valency.

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