CBSE Class 9 Science Notes Chapter 3 Atoms and Molecules

Atoms and Molecules Class 9 is an important chapter in Class 9 Chemistry.

In this chapter, we transition from observing matter in our surroundings to exploring the invisible particles that construct our universe.

These Class 9th Atoms and Molecules Notes break down complex theories into simple sections for quick revision and deep understanding.

What are Atoms And Molecules Class 9​?

Atoms and molecules are the basic building blocks of matter. These Class 10 notes provide a clear understanding of these fundamental particles, exploring their properties, how they interact, and how they form various substances.

Grasping these concepts is important for understanding all chemical reactions and the composition of the world around us.

Atoms And Molecules Class 9​ Notes Detailed

This section provides comprehensive notes on atoms and molecules, breaking down complex ideas into easy-to-understand points for quick revision.

What Is Atom And Molecule?

• Atoms are the smallest units of an element that maintain its chemical identity.

• Molecules are groups of two or more atoms chemically bonded together.

Brief History of the Atom

Before modern science, ancient thinkers proposed the existence of tiny particles:

• Maharishi Kanad (500 BC): An Indian philosopher who suggested that if we keep dividing matter (Padarth), we eventually reach an indivisible particle called Parmanu.

• Ancient Greek Philosophers: Democritus and Leucippus named these indivisible particles "Atoms" (meaning "uncuttable").

What is an Atom?

An Atom is the smallest fundamental particle of an element that can participate in a chemical reaction.

Key Characteristics:

1. Indivisibility: Under standard chemical methods, an atom cannot be divided further.

2. Size: They are incredibly small, measured in nanometers (nm).

• 1nm = 10 power -9m

3. Shape: Atoms are considered to be nearly spherical.

4. Radius of an atom: Measured in nanometres (nm).

• 1 nm = 10⁻⁹ m

• 1 nm = 0.000000001 m

The table below illustrates the scale of atomic and molecular radii compared to everyday objects.

How do Atoms Interact?

Atoms follow specific rules known as the Laws of Chemical Combination. 

1. Law of Conservation of Mass

This law states that mass can neither be created nor destroyed in a chemical reaction.

• Total Mass of Reactants = Total Mass of Products.

2. Law of Constant Proportions

A pure chemical compound always contains the same elements combined in the same fixed proportion by mass, regardless of the source.

• Example: In Water (H2O), the ratio of the mass of Hydrogen to the mass of Oxygen is always 1:8.

Dalton’s Atomic Theory

John Dalton proposed his atomic theory in 1808. This theory provided a foundation for understanding the behavior of matter. His Atomic theory was based on the laws of chemical combination.

Postulates of Dalton’s Atomic Theory:

• All matter is made of tiny particles called atoms.

• Atoms are indivisible (cannot be created or destroyed).

• Atoms of a given element are identical in mass and properties.

• Atoms of different elements have different masses and properties.

Drawbacks of Dalton's Atomic Theory:

1. Dalton's theory stated that atoms were indivisible. However, we now know that atoms are divisible (e.g., by nuclear reactions) and are made of subatomic particles like electrons, protons, and neutrons.

2. Dalton's theory stated that all atoms of a given element have exactly the same mass. But isotopes show that atoms of the same element can have slightly different masses.

3. Dalton's atomic theory said that atoms of different elements have different masses. However, isobars show that atoms of different elements can have the same mass.

Dalton's Symbols of elements or Atoms:

1. Dalton was the first scientist to propose symbols for elements.

2. Initial Naming: Early names were often based on discovery location (e.g., Copper from Cyprus) or properties (e.g., Gold from the English word "yellow").

3. Dalton's symbols were pictorial and proved difficult to draw and use.

Modern Symbols of Elements or Atoms:

The International Union of Pure and Applied Chemistry (IUPAC) standardizes the modern names and symbols of elements. These standardized symbols are essential for universal communication in chemistry.

• Rules for naming:

1. The first letter of the element's name is capitalized (e.g., Hydrogen - H, Oxygen - O, Carbon - C, Phosphorus - P).

2. The first letter is capitalized, and the second letter is lowercase (e.g., Aluminium - Al, Calcium - Ca, Helium - He).

Here are the first 20 elements and their modern symbols. This table serves as a fundamental reference for understanding basic chemical notation.

A list of some other important elements and their symbols. These elements are commonly encountered in various chemical contexts and reactions.

What is a Molecule?

A Molecule is the smallest particle of an element or a compound that is capable of independent existence and shows all the properties of that substance.

What Is The Difference Between An Atom And A Molecule​?

Understanding What is the difference between an atom and a molecule is essential for Class 9 students: 

• Molecules of Elements:

• Molecules of an element are made up of the same type of atoms (e.g., Hydrogen - H₂, Oxygen - O₂).

• Atomicity: The number of atoms present in one molecule of an element.

The following table categorizes elements based on their atomicity. This classification helps in understanding how various elements exist in their stable molecular forms.

• Molecules of compounds:

• Molecules of a compound are made up of different types of atoms. Atoms of different elements combine in fixed proportions to form a molecule, following the Law of Constant Proportions.

• 1. Water (H₂O): Elements: H, O; Mass ratio: 1:8

• 2. Ammonia (NH₃): Elements: N, H; Mass ratio: 14:3

Atomic Mass - Mass of one atom:

Atoms are extremely small, making their mass inconvenient to measure in grams. A specialized unit is used instead.

• A new unit was defined: Atomic Mass Unit (earlier → amu, now → u).

• The mass of one atom of carbon (C-12 isotope) is taken as 12 amu or 12 u.

• 1 amu = (1/12) mass of carbon-12 atom.

• So, 1 atomic mass unit or 1u is defined as 1/12th of the mass of one atom of carbon-12 isotope.

What is Molecular Mass?

The Molecular Mass of a substance is the sum of the atomic masses of all the atoms present in one molecule of that substance. It is also expressed in unified mass (u).

How to calculate it:

1. Write down the chemical formula of the molecule.

2. Identify the number of atoms of each element.

3. Multiply the number of atoms by their respective atomic masses.

4. Add the values together.

Example: Calculating the Molecular Mass of Water (H2O)

• Atomic mass of Hydrogen = 1 u

• Atomic mass of Oxygen = 16 u

• Calculation: (2 * 1) + (1 * 16) = 2 + 16 = 18 u

What are Ions?

Ions are atoms or groups of atoms that carry an electric charge. This charge results from the gain or loss of electrons.

• An atom or group of atoms with an electric charge due to the loss or gain of electrons is called an ion.

• Cation: An atom or group of atoms with a positive charge (due to loss of electrons). (Examples: Sodium ion (Na⁺), potassium ion (K⁺), calcium ion (Ca²⁺), Magnesium ion (Mg²⁺)).

• Anion: An atom or group of atoms with a negative charge (due to gain of electrons). (Examples: Chloride (Cl⁻), oxide (O²⁻)).

• Monoatomic Ion: A single atom with an electric charge (Examples: Na⁺ (sodium ion), Cl⁻ (chloride ion), and S²⁻ (sulfide ion)).

• Polyatomic Ion: A group of atoms with an electric charge (Examples: Ammonium cation (NH₄⁺), Carbonate anion (CO₃²⁻)).

What is Valency?

Valency helps us predict how atoms will combine to form molecules. It represents the combining power of an element.

1. The combining capacity of an element is called its valency.

2. The valency of an element can be thought of as the "hands" of that atom, indicating how many bonds it can form.

3. It determines how one element will combine with other elements.

• Example: The valency of Magnesium is 2, and the valency of chlorine is 1. This results in the compound MgCl₂.

Atoms and Molecules Class 9 Notes PDF

Downloading the Atoms and Molecules Class 9 Notes PDF is the most effective way to ensure you have effective study material.

The PDF notes are organized to mirror the NCERT curriculum. It provides a structured flow that helps students move from basic definitions to complex chemical formulas easily. Below is the Atoms and Molecules Class 9 Notes PDF created by PW faculty:

Class 9 Science Notes Chapter 3 PDF