ICSE Class 8 Chemistry Atomic Structure Notes | PDF Download

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Introduction

Atomic Structure is one of the most important chapters in ICSE Class 8 Chemistry. This chapter helps students understand the basic structure of matter, atoms, subatomic particles, and electronic arrangement. These notes are prepared in simple language for quick revision and exam preparation.
Students can use these ICSE Class 8 Chemistry Atomic Structure Notes PDF for school exams, homework, and last-minute revision.

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Atomic Structure | ICSE Class 8 Chemistry Notes

Atom

An atom is the smallest particle of an element that exhibits all the properties of that element.

Atoms are extremely tiny particles and cannot be seen with the naked eye.

They can be observed using powerful electron microscopes.

Atoms take part in chemical reactions.

Dalton’s Atomic Theory

John Dalton proposed that:

All matter is made up of very small particles called atoms.
Atoms are indivisible and cannot be created or destroyed in a chemical reaction.
All atoms of the same element are identical in mass and properties.
Atoms of different elements have different masses and properties.
Atoms combine in simple whole number ratios to form compounds.
Atoms rearrange during chemical reactions.

Limitations / Drawbacks of Dalton’s Atomic Theory

Dalton stated that atoms are indivisible particles. However, later discoveries showed that atoms are made up of smaller particles called electrons, protons, and neutrons.
Dalton proposed that all atoms of an element have exactly the same mass. But it is now known that atoms of the same element can have slightly different masses, known as isotopes.

Sub-Atomic (Fundamental) Particles

Atoms are made up of three main particles:

Electron (e⁻): Negatively charged particle
Proton (p⁺): Positively charged particle
Neutron (n⁰): Neutral particle (no charge).

Nucleons

Protons and neutrons together are called nucleons.

Electrical Neutrality of an Atom

An atom has the same number of protons and electrons.
The positive charge of protons is balanced by the negative charge of electrons.
As a result, the overall charge of an atom is zero, making it electrically neutral.

Discovery of Electrons : Cathode Rays (By J. J. Thomson)

ICSE Class 8 Chemistry Chapter 4 Atomic Structure

Electrons were discovered by J.J. Thomson while he was studying the properties of cathode rays in a discharge tube.

The Experiment: High-voltage electricity was passed through a glass tube (discharge tube) containing gas at very low pressure.
Observation: Streams of rays were emitted from the negative electrode (cathode) toward the positive electrode (anode). These were named cathode rays.
Conclusion: These rays consisted of negatively charged particles, which Thomson named electrons.

Properties of Electrons

Negatively charged particle.
Present in all atoms.
Mass of electron = 9.108 × 10⁻³¹ kg
It is 1/1837 of the mass of a hydrogen atom.
Charge of electron = − 1.602 × 10⁻³¹ C

Discovery of Protons – Anode Rays/Canal Rays (By E. Goldstein)

Experiment:

Used perforated cathode discharge tube.
Observed canal rays (anode rays).

Conclusion:
Canal rays consist of positively charged particles called protons.

Properties of Protons

Positively charged particle.
Charge = +1.602 × 10⁻¹⁹ C.
Mass ≈ 1.67 × 10⁻²⁷ kg.
Present in the nucleus of atoms.

Discovery of Neutrons (By J. Chadwick)

Conclusion:
Discovered neutral particles called neutrons.

Properties of Neutrons

A neutron is electrically neutral (no charge).

Mass slightly greater than proton.
Present in the nucleus.
The number of neutrons may be equal to or greater than the number of protons.

Atoms of the same element can have different numbers of neutrons, forming isotopes.

Comparison between Proton, Neutron and Electron

Particles	Location	Mass	Charge	Discovered by
Electron (e⁻)	Orbit	Negligible	−1.6 × 10⁻¹⁹ C	J. J. Thomson
Proton (p⁺)	Nucleus	1.67 × 10⁻²⁷ kg	+1.6 × 10⁻¹⁹ C	E. Rutherford
Neutron (n⁰)	Nucleus	1.67 × 10⁻²⁷ kg	No charge	J. Chadwick

Thomson’s Atomic Model (Plum Pudding Model)

Proposed by J. J. Thomson.

Features:

Atom is a positively charged sphere.
Electrons embedded inside it like plums in pudding.
Atom is electrically neutral.

Limitation:
Could not explain experimental observations.

Rutherford’s Atomic Model

Based on Alpha Particle Scattering Experiment (1911).

ICSE Class 8 Chemistry Chapter 4 Atomic Structure img11

Observations of Rutherford’s Experiment

Rutherford observed the following results:

Most alpha particles passed straight through the gold foil without any deflection.
Some alpha particles were deflected slightly from their path.
Very few alpha particles bounced back.

Conclusions of Rutherford

Empty Space: Since most particles passed through, most of the space inside an atom is empty.
Central Mass: There is a heavy, positively charged mass at the center that caused the deflection.
The Nucleus: This central mass is very small compared to the total size of the atom. Rutherford named this dense center the nucleus.

Bohr’s Atomic Model (1913)

Proposed by Niels Bohr.

Main Points:

Electrons revolve in fixed circular paths called orbits or shells.
Each orbit has fixed energy.
Shells are named K, L, M, N.

ICSE Class 8 Chemistry Chapter 4 Atomic Structure img12

Energy increases with distance from nucleus.

Shell nearest to nucleus → lowest energy
Shell farthest from nucleus → highest energy

Structure of an Atom

An atom has two main parts:

1. Nucleus

Contains protons and neutrons
Positively charged

2. Electron shells

Surround the nucleus
Contain electrons

Atomic Number (Z)

The number of protons present in the nucleus of an atom is called the atomic number. It is represented by Z.

Atomic number = Number of protons
Atomic number = Number of electrons
Example: Oxygen
Z = 8

Mass Number (A)

The total number of protons and neutrons present in the nucleus is called the mass number. It is represented by A.

Mass Number (A) = Protons + Neutrons
Mass number = Atomic number + No. of neutrons
No. of neutrons = Mass number − Atomic number
Example:
Sodium \(\left({_11^{23}}Na\right)\)
A = 23
Z = 11
Neutrons = 23 − 11 = 12

Electronic Configuration

The arrangement of electrons in different shells (orbits) of an atom is called electronic configuration.

Bohr-Bury Rules

Maximum electrons in a shell = 2n²

Shell	n	Maximum Electrons
K	1	2
L	2	8
M	3	18
N	4	32

Symbolic Representation of Elements

ICSE Class 8 Chemistry Chapter 4 Atomic Structure img13

Outer shell cannot contain more than 8 electrons.

Valence Shell & Valency

Valence Shell : The outermost shell of an atom.
Valence Electrons : Electrons present in the outermost shell.
Valency : The combining capacity of an atom.
Example:
Na → valency = 1
O → valency = 2
N → valency = 3

Octet Rule

Atoms tend to gain, lose, or share electrons to achieve 8 electrons in their outermost shell.
This stable condition is called the Octet Rule.

Example

Neon (2,8) is stable because its outer shell has 8 electrons.

Hydrogen and Helium follow the duplet rule (2 electrons).

Ions

Charged particles formed when atoms gain or lose electrons.

Cation : Positive ion (loss of electron)
Example:
Na → Na⁺
Anion : Negative ion (gain of electron)
Example:
Cl → Cl⁻

Isotopes

Isotopes are atoms of same elements having same atomic number and different mass numbers.

Examples of Isotopes:

1. Hydrogen: Has three isotopes:

Protium \(\left({_\mathbf{1}^\mathbf{1}}\mathbf{H}\right)\) : 1 proton, 0 neutrons.
Deuterium \(\left({_\mathbf{1}^\mathbf{2}}\mathbf{H}\right)\) : 1 proton, 1 neutron.
Tritium \(\left({_\mathbf{1}^\mathbf{3}}\mathbf{H}\right)\) : 1 proton, 2 neutrons.

2. Carbon: Carbon-12 \(\left({_\mathbf{6}^\mathbf{12}}\mathbf{C}\right)\), Carbon-13 \(\left({_\mathbf{6}^\mathbf{13}}\mathbf{C}\right)\), and Carbon-14 \(\left({_\mathbf{6}^\mathbf{14}}\mathbf{C}\right)\).

3. Chlorine: Chlorine-35 \(\left({_\mathbf{17}^\mathbf{35}}\mathbf{Cl}\right)\) and Chlorine-37 \(\left({_\mathbf{17}^\mathbf{37}}\mathbf{Cl}\right)\).

Properties of Isotopes

Same chemical properties
Different physical properties

Radicals

A radical is an atom or group of atoms that carries a positive or negative charge.
Examples:

Radical	Formula	Valency
Ammonium	NH₄⁺	1
Nitrate	NO₃⁻	1
Sulphate	SO₄²⁻	2
Carbonate	CO₃²⁻	2
Phosphate	PO₄³⁻	3

Variable Valency

Some elements show more than one valency.

Lower valency → suffix “ous”
Higher valency → suffix “ic”

Examples:

Ion	Name	Valency
Fe²⁺	Ferrous [Iron (II)]	2
Fe³⁺	Ferric [Iron (III)]	3
Cu⁺	Cuprous [Copper (I)]	1
Cu²⁺	Cupric [Copper (II)]	2

Electronic Configurations of First Twenty Elements

Download ICSE Class 8 Chemistry Atomic Structure Notes PDF

Students looking for ICSE Class 8 Chemistry Atomic Structure Notes PDF Download can use these notes for quick revision and concept clarity. These notes cover atom structure, subatomic particles, electronic configuration, atomic number, mass number, and valency in an easy format.