CBSE Class 9 Science Notes Chapter 4 Structure of the Atom

Preparing through the Structure of the Atom Class 9 Notes can help students understand how the universe is constructed. Initially, scientists believed atoms were indivisible, but through decades of experimentation, we now know they are composed of complex sub-atomic particles.

Class 9 Chemistry Chapter 4 Notes are helpful in understanding the basic structure of an atom and its discovery as well. These notes are created by the PW faculty to help students understand the basics and prepare well for Class 9 exams.

Class 9 Chemistry Chapter 4 Notes

Structure Of Atom Class 9 Notes explore the internal world of the atom, covering fundamental sub-atomic particles and the evolution of atomic models from Thomson to Bohr. 

The Discovery of Sub-Atomic Particles

Before reading the Structure Of Atom Class 9 Notes, you must know that the atoms are divisible. The discovery of electrons and protons changed chemistry forever.

• Canal Rays (1886): Discovered by E. Goldstein, these are positively charged radiations that led to the discovery of protons.

• Cathode Rays (1897): J.J. Thomson discovered negatively charged particles called electrons using cathode ray tubes.

• Neutrons (1932): James Chadwick discovered a neutral particle in the nucleus called the neutron, which has a mass nearly equal to a proton.

Early Atomic Models

Below are the early atomic models which were discovered by scientists like Thomson and Rutherford:

 Thomson’s Model of an Atom

Thomson proposed that an atom resembles a "Plum Pudding" or a watermelon.

• Thomson's Proposals:

• An atom consists of a positively charged sphere with electrons embedded in it.

• The negative and positive charges are equal in magnitude, making the atom electrically neutral overall.

• This model failed to explain the results of later experiments, particularly Rutherford's Gold foil experiment, leading to its rejection.

Rutherford’s Alpha Particle Scattering Experiment

Rutherford’s experiment shifted our understanding of whats the structure of an atom. He bombarded a thin gold foil with alpha particles. It was conducted in the year 1909 by Greiger and Marsden under the direction of Ernst Rutherford.

• Observations: Some α-particles were deflected by small angles. Very few α-particles (about one out of every 12,000) rebounded, meaning they were deflected by 180°.

• Conclusions: He discovered the Nucleus—a tiny, dense, positively charged center where all the mass of the atom resides. The nucleus is very small. A very small fraction of α-particles deflected by 180° indicated that the entire positive charge and mass are concentrated in a tiny space. The radius of the Nucleus is about 10⁵ times less than the radius of the atom.

• Drawback: He couldn't explain why revolving electrons don't lose energy and crash into the nucleus.

What is Bohr’s Model?

Niels Bohr solved the stability problem in Structure of the Atom Class 9 Notes. It improved the Rutherford's model by proposing:

1. Electrons revolve in fixed orbits called Discrete Orbits or Energy Shells.

2. Shells are represented as K, L, M, N or n = 1, 2, 3, 4.

3. While revolving in a particular Energy level, an electron maintains a fixed energy. It neither loses nor gains energy in the form of radiation.

Discovery of Neutron and Updated Atomic Model

The discovery of the neutron further refined the atomic model.

• Year: 1932

• Discovery: James Chadwick discovered the Neutron.

• Neutron Properties:

• Has zero charge.

• Mass is almost equal to that of a proton.

• Neutrons are present in the Nucleus along with protons.

Atomic Number (Z)

The atomic number is a fundamental property that defines an element.

• Definition: The number of protons in the Nucleus of an atom is called its Atomic Number (Z).

• Atomic Number (Z) is the unique identity of an element.

• If the Atomic Number changes, the element changes.

• For a neutral atom: Z = number of protons = number of electrons in the atom.

First Twenty Elements Along with Their Valency

The table below lists the first twenty elements, including their symbols, atomic numbers, and electron distribution in shells, along with their valency.

Electronic Configuration and Valency

The arrangement of electrons is governed by the Bohr-Bury Rule:

• Formula: The maximum number of electrons in a shell is (K=2, L=8, M=18).

• Octet Rule: Atoms are most stable when their outermost shell has 8 electrons.

• Valency: The combining capacity of an atom (electrons lost, gained, or shared to complete an octet).

Atomic Properties

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Sum of protons and neutrons.

• Atomic Packing Factor: While the atomic packing factor of BCC structure is a higher-level concept ($0.68$), for Class 9, focus on how mass is concentrated in the nucleus.

Octet and Duplet Rules for Stability

These rules explain why atoms react and form bonds.

1. Octet Rule: Atoms tend to react in a way that allows them to achieve 8 electrons in their outermost valence shell, leading to chemical stability.

2. Duplet Rule: For very light elements like Hydrogen and Helium, stability is achieved with 2 electrons in their outermost (and only) shell.

What is Valency?

Valency determines an element's combining capacity.

• Definition: The number of electrons gained, lost, or shared by an atom to achieve an octet (8 electrons) in its outermost shell is called valency.

• Exceptions: Elements like H, He, Li, Be, B aim for 2 electrons (duplet rule).

• Ion Formation: When atoms lose or gain electrons, they form charged particles called ions.

What is Mass Number (A)?

The mass number helps in identifying isotopes and calculating the number of neutrons.

• Definition: The sum of the total number of protons and the total number of neutrons in the nucleus of an atom.

• Mass Number (A) = number of protons + number of neutrons = Number of Nucleons.

• Relationship between Mass No. (A) and Atomic No. (Z):

• A = Number of protons + Number of neutrons

• Z = Number of protons

• Therefore, A - Z = number of neutrons.

• Representation: An element is typically represented as (A / Z) Element.

• Example: For (12 / 6) Element, the number of Neutrons = 12 - 6 = 6.

What are Isotopes and Isobars?

• Isotopes: Atoms of the same element with the same Atomic Number but different Mass Numbers (e.g., Protium, Deuterium, and Tritium)

• Isobars: Atoms of different elements with different Atomic Numbers but the same Mass Number (e.g., Calcium and Argon).

Structure of the Atom Class 9 Notes PDF

Chapter 4 The Structure Of An Atom​ Class 9 Notes PDF is designed to help students prepare well for Class 9 exams. It includes all the essential chemical formulas, valency tables, and labeled diagrams of atomic models.

Furthermore, a Structure of the Atom Class 9 Notes PDF provides a structured breakdown of the Bohr-Bury rules and isotope applications. This makes it a useful resource for last-minute revisions. Below is the Structure of the Atom Class 9 Notes PDF created by the PW faculty: 

Structure of the Atom Class 9 Notes PDF