Chapter-Journey Inside the Atom Science Exploration class 9 in english Medium CBSE Notes

Chapter 8. Journey Inside the Atom

This section explains the discovery of the neutron, the three fundamental subatomic particles, atomic number, mass number and the standard representation of an atom. These concepts help us understand the composition of atoms and the identity of different elements. 

Discovery of the Neutron

Although protons explained the positive charge of the nucleus, they could not explain the actual mass of atoms. In 1932, James Chadwick discovered another subatomic particle called the neutron, which has no electric charge but contributes to the mass of the atom. 

Definition

A neutron is an electrically neutral subatomic particle present in the nucleus of an atom.

Importance of the Discovery

• Explained Atomic Mass – Neutrons account for the extra mass of atoms.
• Completed the Structure of the Atom – Scientists identified all three fundamental subatomic particles.
• Explained Nuclear Stability – Neutrons help reduce the repulsion between positively charged protons.
• Advanced Nuclear Science – The discovery led to the development of nuclear energy and modern atomic research.

Subatomic Particles

An atom consists of three fundamental particles—electrons, protons and neutrons. Each particle has its own position, charge and contribution to the mass of the atom.

Subatomic Particle	Symbol	Charge	Location
Electron	e−	−1	Outside the nucleus
Proton	p+	+1	Inside the nucleus
Neutron	n0	0	Inside the nucleus

 

Atomic Number (Z)

The atomic number is one of the most important characteristics of an element because it identifies the element.

Definition

The atomic number (Z) is the number of protons present in the nucleus of an atom.

Important Facts

• Every element has a unique atomic number.
• In a neutral atom, the number of electrons is equal to the number of protons.
• The atomic number determines the identity and chemical behaviour of an element.

 

Mass Number (A)

The mass of an atom mainly depends on the particles present inside its nucleus.

Definition

The mass number (A) is the total number of protons and neutrons present in the nucleus of an atom.

Formula

Mass Number (A) = Number of Protons + Number of Neutrons

A = p + n

 

Difference Between Atomic Number and Mass Number

Atomic Number (Z)	Mass Number (A)
Number of protons.	Total number of protons and neutrons.
Determines the identity of an element.	Represents the total mass of the nucleus.
Remains fixed for an element.	May vary due to different numbers of neutrons.

Standard Representation of an Atom

An atom is represented by writing its mass number at the upper left and its atomic number at the lower left of the chemical symbol.

General Representation

^A_ZX

Where:

• X – Symbol of the element
• A – Mass number
• Z – Atomic number

Example: Carbon is represented as ¹²₆C. 

Relationship Among Protons, Neutrons and Electrons

• Protons = Atomic Number
• Electrons = Protons (for a neutral atom)
• Neutrons = Mass Number − Atomic Number

Solved Examples

Element	Atomic Number	Mass Number	Neutrons
Hydrogen	1	1	0
Helium	2	4	2
Lithium	3	7	4

ATP Education Concept Builder

Always remember that the atomic number identifies the element, while the mass number tells the total number of particles present in the nucleus. If the atomic number and mass number are known, the number of neutrons can easily be calculated using the formula Neutrons = A − Z.

ATP Education Exam Booster

• Remember the symbols, charges and positions of electrons, protons and neutrons.
• Learn the definitions of atomic number and mass number.
• Memorise the formulas: A = p + n and Neutrons = A − Z.
• Practise writing the standard notation of atoms.
• Solve numerical questions based on atomic number, mass number and neutrons.
• CBSE competency-based questions frequently ask students to calculate protons, neutrons and electrons from the given atomic or mass number.

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