NCERT Curiosity · Class 9 · Science
Journey Inside the Atom
Electrons are pulled toward the positive nucleus, yet they never fall in — the puzzle that took four scientists and over a century to solve.
Atomic Models Subatomic Particles Isotopes & Isobars Valency
Key idea
Rediscovering the Roots of Atomic Theory
A brilliant guess

Over 2,000 years ago, Kanada in India and Democritus in Greece imagined matter divided until reaching indivisible parmanus, or atomos.

A scientific start

Dalton's atomic theory (1808) was the first description built on real experiments — all matter is made of indivisible atoms.

Key idea
Cathode rays were streams of tiny, negatively charged particles.
In 1897, J. J. Thomson passed electric current through low-pressure gas and found these rays in every element he tested — naming them electrons, and proving atoms have smaller parts after all.
Key idea
A sphere of positive charge, studded with electrons.
Thomson's model, like a watermelon: the positively charged pulp is the sphere, and electrons are seeds scattered through it — the first real attempt to balance an atom's charges.
Key idea
The Gold Foil Experiment
Most passed straight through
Most of the atom is empty space
→
A few were sharply deflected
Something small and dense repelled them
→
A very few bounced straight back
That something carries concentrated positive charge
Key idea
Mostly empty space, with a tiny, dense nucleus at the centre.
If an atom were the size of a cricket ground, its nucleus would be a single pepper grain at the centre — carrying almost all the mass, with electrons orbiting far outside it.
Key idea
Bohr Solves the Stability Problem

Fixed shells

Electrons follow set circular paths — K, L, M, N — not random ones.

No energy loss

While an electron stays in an allowed shell, it never radiates energy or spirals inward.

Jumps need fixed energy

Moving shells means absorbing or releasing an exact amount of energy.

Key idea
The Subatomic Particles

Electron

Charge −1 · orbits the nucleus.

Proton

Charge +1 · inside the nucleus.

Neutron

Charge 0 · inside the nucleus.

Key idea
Atomic number (Z)

The number of protons in the nucleus — equal to the electrons in a neutral atom. It uniquely identifies an element.

Mass number (A)

Protons + neutrons (nucleons). Carbon: atomic number 6, mass number 12. Each shell holds at most 2n² electrons, and the outermost shell holds at most 8.

Key idea
Valency: The Combining Capacity

Sodium (2,8,1)

Loses 1 electron — valency 1.

Oxygen (2,6)

Gains 2 electrons — valency 2.

Carbon (2,4)

Shares 4 electrons — valency 4.

Key idea
Isotopes

Same atomic number, different mass number. Hydrogen's three isotopes all share 1 proton, but 0, 1, or 2 neutrons.

Isobars

Same mass number, different atomic number. Calcium, potassium, and argon all total 40 nucleons.

Key idea
The Journey of Atomic Models
Dalton
Indivisible particle
→
Thomson
Charges in a sphere
→
Rutherford
Dense nucleus
→
Bohr
Fixed energy levels
Chapter · key terms to remember
Key Terms

Nucleus

The tiny, dense, positively charged centre of an atom.

Atomic number (Z)

Number of protons — identifies the element.

Mass number (A)

Total protons + neutrons (nucleons).

Chapter · key terms to remember
More Key Terms

Electronic configuration

How electrons are distributed among shells.

Valency

Electrons gained, lost, or shared to complete an octet.

Isotopes

Same atomic number, different mass number.

Chapter · key terms to remember
More Key Terms

Isobars

Same mass number, different atomic number.

Electron / Proton / Neutron

Charge −1 orbiting / +1 in nucleus / 0 in nucleus.

Opening

Jump to a section — tap anywhere else to close