NCERT Class 9 Science Chapter 3 Notes - Atoms And Molecules PDF Notes

Have you ever wondered how things around us are made? Like the air we breathe, the phone we are currently using to read this article, or the food we eat daily to quench our appetite. The answer to all those questions is atoms and molecules. Everything in our environment, from the tiniest grain of sand to the vast universe, is made up of atoms and molecules. Atoms are the smallest unit of matter while molecules are made up of a combination of two or more atoms and they are so small that we cannot see them from the naked eye, to see them we require a high-end microscope. Even a small drop of water contains countless molecules. Just as Bricks combine together to make huge structures, atoms combine to form molecules, which make everything around us.

This Story also Contains

1. NCERT Class 9 Chapter 3 Class Notes
2. Atoms and Molecules
3. Laws of Chemical Combination
4. What are the modern-day symbols of atoms of different elements?
5. What is a Molecule?
6. How to Write Chemical Formulae
7. Molecular Mass and Mole Concept
8. Subject Wise NCERT Exemplar Solutions
9. Subject wise NCERT Solutions

NCERT Notes for Class 9 of Atoms and Molecules are created by experienced subject experts, in a very comprehensive and systematic way covering all the important topics of atoms and molecules. The information on the structure and behaviour of atoms and molecules helps us understand chemical reactions, the composition of materials and as well as biological processes. These notes provide a clear and detailed understanding of these fundamental topics. Concepts like the laws of chemical combination, atomic mass, molecular mass, and chemical formulas are covered in this chapter that will help students build a strong foundation in chemistry. These notes adhere to the CBSE-recommended NCERT Class 9 Science syllabus.

Also, students can refer to

NCERT Class 9 Chapter 3 Class Notes

Topics covered in this Chapter are

Atoms and Molecules

Definition: Democritus explained that if we keep on dividing a matter, we will end up with its smallest unit, which won’t be divisible further, and he named it an atom.

Laws of Chemical Combination

Antoine L. Lavoisier is called the father of chemistry, for he gave two important laws of chemical combination which laid the foundation of chemical sciences, and the laws are as follows:

Law of Conservation of Mass

It states that “Mass is not created and not destroyed, in a chemical reaction”.

Law of Constant Proportions

“The elements are present in definite proportions by mass, in a chemical substance”.

Explanation of these laws as given by Dalton (Dalton’s Atomic Theory).

The postulates of the theory are given below:

1. “Matter is made of very small particles called atoms”.

2. “Atoms are not divisible particles, which are not created and not destroyed in a chemical reaction”.

3. “Atoms of the same element have identical mass and chemical properties”.

4. “The chemical properties of the atoms are different for different elements.”.

5. “Compounds are formed by combining atoms of different elements in ratios of small whole numbers”.

How big are atoms?

The relative size of different matters:

What are the modern-day symbols of atoms of different elements?

Some elements and their symbols:

Atomic Number

It is the number of protons present in an element.

Atomic Mass

It is the number of protons and neutrons present in an element.

Carbon-12 isotope is the standard reference for calculating atomic masses of other elements. One atomic mass unit is a mass unit equal to one-twelfth (1/12th) the mass of one atom of carbon-12.

What is a Molecule?

It is a group of two or more elements held together in a chemical bond by attractive forces.

Atomicity

The number of atoms in a molecule is called the atomicity of the molecule.

• For example:, argon and helium are monoatomic,
• Oxygen and chlorine are diatomic.
• Phosphorus is tetra-atomic and
• Sulphur is polyatomic.

Ion

An ion is a charged particle; it can be positively charged or negatively charged. Positively charged ions is called ‘cation’ and negatively charged ion is called ‘anions’.

How to Write Chemical Formulae

It is a symbolic representation of a compound and its composition.

Given below are the rules for writing a chemical formula:

• The charges on the ions have to balance each other.

• If the compound contains a metal and a nonmetal, the metal is written first.

For example: sodium chloride (NaCl), where chlorine is a non-metal, so written on the right, and sodium is a metal, so written on the left.

• In compounds with polyatomic ions, the ion is enclosed in a bracket and then the number of the ion is indicated. If number of polyatomic ion is one, the bracket is not required.

For example: The formula for aluminium oxide is given by ${\mathrm{Al}}_2{\mathrm{O}}_3$, as Al has 3+ charge and O has 2- charge.

Molecular Mass and Mole Concept

The molecular mass of a substance is equal to the sum of the atomic masses of all the atoms in that molecule.

If the substance contains ions as its constituents, then the molecular mass is called Formula Unit Mass.

The number of particles present in 1 mole of any substance is given by Avogadro Number, which is 6.022 × 1023.

Number of moles $=\frac{\text{ Given Mass }}{\text{ Molar Mass }}=\frac{\text{ given number of particles }}{\text{ AvogadroNumber }}$

Mole Concepts

Any quantity in number having a mass equal to its atomic or molecular mass in grammes is referred to as one mole of any species (atoms, molecules, ions, or particles). A single mole of any given substance comprises 6.022 × 1023 particles, which can be atoms, molecules, or ions. This is an experimentally obtained value. Named in honour of the Italian scientist Amedeo Avogadro, this number is also known as the Avogadro Constant or Avogadro Number (represented by N0).

1 mole (of anything) = 6.022 × 1023 in number

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