notes on ch structure of atom class 9 ncert

Class 9 Science: Structure of Atom Chapter Notes

1. What is an Atom?

An atom is the smallest unit of matter that retains the properties of an element. It consists of three subatomic particles: protons, neutrons, and electrons.

2. Subatomic Particles

An atom is made up of the following subatomic particles:

• Protons: Positively charged particles, found in the nucleus of the atom.
• Neutrons: Neutral particles (no charge), also located in the nucleus.
• Electrons: Negatively charged particles, revolve around the nucleus in specific orbits or shells.

3. Atomic Models

A. Dalton's Atomic Theory

• Proposed by John Dalton in 1808.
• Stated that atoms are indivisible particles that cannot be created or destroyed.
• All atoms of a given element are identical in mass and properties.

B. J.J. Thomson's Model (Plum Pudding Model)

• Proposed by J.J. Thomson in 1897.
• Described the atom as a sphere of positive charge with negatively charged electrons embedded in it, like "plums in a pudding."

C. Rutherford's Model

• Proposed by Ernest Rutherford in 1911 after his gold foil experiment.
• He discovered the nucleus, which contains positively charged protons.
• Electrons revolve around the nucleus in orbits.
• Most of the atom is empty space.

D. Bohr's Model

• Proposed by Niels Bohr in 1913.
• Electrons revolve around the nucleus in fixed orbits (energy levels or shells).
• Each orbit corresponds to a specific energy level. Electrons do not lose energy while moving in their fixed orbits.

4. Atomic Number and Mass Number

• Atomic Number (Z): The number of protons in the nucleus of an atom. It also equals the number of electrons in a neutral atom.
• Mass Number (A): The total number of protons and neutrons in the nucleus of an atom.

5. Isotopes and Isobars

A. Isotopes

Isotopes are atoms of the same element that have the same atomic number but different mass numbers. For example:

• Hydrogen Isotopes: Protium (¹H), Deuterium (²H), Tritium (³H).
• They have the same number of protons but different numbers of neutrons.

B. Isobars

Isobars are atoms of different elements that have the same mass number but different atomic numbers. For example:

• Calcium-40 (⁴⁰Ca) and Argon-40 (⁴⁰Ar) are isobars.
• They have the same mass number but different chemical properties.

6. Electronic Configuration

Electrons are arranged in different energy levels or shells around the nucleus. The distribution of electrons in the various orbits is known as the electronic configuration.

• First shell (K): Can hold a maximum of 2 electrons.
• Second shell (L): Can hold a maximum of 8 electrons.
• Third shell (M): Can hold a maximum of 18 electrons.

7. Valency

The valency of an element is the combining capacity of its atoms to form molecules. It is determined by the number of electrons in the outermost shell (valence electrons).

8. Distribution of Electrons in an Atom

Element	Atomic Number (Z)	Electronic Configuration	Valency
Hydrogen	1	1	1
Helium	2	2	0
Oxygen	8	2, 6	2
Sodium	11	2, 8, 1	1
Chlorine	17	2, 8, 7	1