Science · Class 9 · Chapter 8

Journey Inside the Atom

• Electron / Proton / Neutron — Charge −1, moving around nucleus / +1, in nucleus / 0, in nucleus.
• 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).
• 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.
• Isobars — Same mass number, different atomic number.

Chapter 8 · Concept 1 of 18

Rediscovering the Roots of Atomic Theory

• Parmanu and atomos — Over 2,000 years ago, Acharya Kanada (India) and Leucippus and Democritus (Greece) imagined matter divided until reaching indivisible particles — parmanus, or atomos. A brilliant guess, not an experiment.
• Dalton's atomic theory (1808) — The first scientific description: all matter is made of indivisible atoms, based on real experiments — the starting point for our modern understanding.

Chapter 8 · Concept 2 of 18

J. J. Thomson Discovers the Electron

• In 1897, Thomson studied electric current through low-pressure gas, and found rays flowing from the cathode to the anode.
• Negatively charged, and inside every atom — These cathode rays were streams of tiny, negatively charged particles — electrons — present in every element, proving atoms have smaller parts after all.

Chapter 8 · Concept 3 of 18

Thomson's Model of the Atom

• If atoms are neutral, where is the balancing positive charge?
• A sphere of positive charge, studded with electrons — Like a watermelon — the positively charged pulp is the sphere, and the electrons are seeds scattered through it. The first real attempt to balance an atom's charges.

Chapter 8 · Concept 4 of 18

The Gold Foil Experiment

• Observation | What it meant
• Most α-particles passed straight through the foil — 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 a concentrated positive charge

Chapter 8 · Concept 5 of 18

Rutherford's Nuclear Model

• A tiny, dense nucleus — Positive charge and most of the mass are packed into a nucleus about 100,000 times smaller than the atom itself. Electrons orbit it like planets around the Sun.
• Mostly empty space — If an atom were the size of a cricket ground (~100 m across), its nucleus would be a single pepper grain at the centre.

Chapter 8 · Concept 6 of 18

A Missing Piece: Why Don't Atoms Collapse?

• The stability problem — An orbiting electron is constantly accelerating (changing direction), so classically it should radiate energy, spiral inward, and crash into the nucleus. But atoms are stable — this model couldn't explain why.
• The proton, named by Rutherford — The nucleus's positive charge comes from protons — heavier than electrons, with an equal and opposite charge. A neutral atom has equal protons and electrons.

Chapter 8 · Concept 7 of 18

Bohr's Model of the Atom (1913)

• Rule | Meaning
• Fixed shells (K, L, M, N…) — Electrons follow set circular paths, or energy levels — not random paths
• No energy loss in a shell — While in an allowed shell, an electron doesn't lose energy
• Energy rises with distance — K (closest, n=1) has the least energy; farther shells have more
• Jumps need fixed energy — Moving shells means absorbing or releasing an exact amount of energy

Chapter 8 · Concept 8 of 18

James Chadwick Discovers the Neutron (1932)

• A helium atom has 2 protons — but its mass is about 4 times a hydrogen atom's, not double. Something else was adding mass.
• Same mass as a proton, but no charge — The neutron explained the missing mass. Found in the nucleus of every atom except ordinary hydrogen, neutrons also help hold the nucleus together against proton-proton repulsion.

Chapter 8 · Concept 9 of 18

The Subatomic Particles

• Particle | Symbol | Relative charge | Found where
• Electron — e⁻ — −1 — Orbiting the nucleus
• Proton — p⁺ — +1 — Inside the nucleus
• Neutron — n⁰ — 0 — Inside the nucleus

Chapter 8 · Concept 10 of 18

Symbols of Elements

• From pictures to letters — Dalton first drew pictorial symbols (1803). Berzelius (1813) proposed Latin-based letter symbols — now standardised worldwide by IUPAC.
• The rules — First letter capital, second (if any) lowercase — hydrogen H, aluminium Al, not AL. Some come from Latin/Greek/German names: iron is Fe (ferrum), mercury Hg (hydrargyros), tungsten W (wolfram).

Chapter 8 · Concept 11 of 18

Atomic Number and Mass Number

• Atomic number (Z) — The number of protons in the nucleus — equal to the number of electrons in a neutral atom. It uniquely identifies an element.
• Mass number (A) — The total number of protons and neutrons (nucleons). Mass number = protons + neutrons. Carbon: ¹²₆C — atomic number 6, mass number 12.

Chapter 8 · Concept 12 of 18

How Electrons Fill Energy Levels

• Rule | Detail
• Maximum electrons per shell — 2n² — K holds 2, L holds 8, M holds 18
• Maximum in the outermost shell — 8 electrons (or just 2, if it's the only shell)
• Filling order — Innermost first: K, then L, then M… never skip ahead

Chapter 8 · Concept 13 of 18

Electron Distribution: A Few Examples

• Element | Z | K | L | M
• Hydrogen (H) — 1 — 1 — – — –
• Helium (He) — 2 — 2 — – — –
• Carbon (C) — 6 — 2 — 4 — –
• Neon (Ne) — 10 — 2 — 8 — –
• Sodium (Na) — 11 — 2 — 8 — 1
• Chlorine (Cl) — 17 — 2 — 8 — 7

Chapter 8 · Concept 14 of 18

Valency: The Combining Capacity

• Atoms with a full outermost shell (an octet of 8, or 2 for helium) are stable. Others lose, gain, or share electrons to get there.
• Element | Valence electrons | Tendency | Valency
• Sodium (2, 8, 1) — 1 — Loses 1 electron — 1
• Oxygen (2, 6) — 6 — Gains 2 electrons — 2
• Carbon (2, 4) — 4 — Shares 4 electrons — 4

Chapter 8 · Concept 15 of 18

Isotopes

• Same atomic number, different mass number — atoms of the same element with a different number of neutrons.
• Hydrogen's three isotopes — Protium (no neutrons, ~99.98%), deuterium (1 neutron), and tritium (2 neutrons) — all with 1 proton and 1 electron, so all share the same chemical properties.

Chapter 8 · Concept 16 of 18

Average Atomic Mass

• Chlorine's isotopes: 35 u (about 75%) and 37 u (about 25%). A simple average ignores how common each one actually is.
• Weighted average = 35.5 u — (35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5 u — reflecting that ³⁵Cl is three times more common than ³⁷Cl in nature.

Chapter 8 · Concept 17 of 18

Isobars

• Different elements can share the same mass number, despite having different atomic numbers.
• Calcium, potassium, and argon: all mass number 40 — Calcium (Z=20), potassium (Z=19), and argon (Z=18) each have 40 nucleons total, despite different numbers of protons — they are isobars, not isotopes.

Chapter 8 · Concept 18 of 18

The Journey of Atomic Models

• Dalton Indivisible particle
• Thomson Charges embedded in a sphere
• Rutherford Dense nucleus, mostly empty
• Bohr Fixed energy levels
• Modern Electron clouds — still evolving

Quick Recap

Check Your Understanding

1. 1Three atomic species X, Y, Z have: X (18 protons, 19 neutrons), Y (17 protons, 18 neutrons), Z (17 protons, 20 neutrons). Explain the relation between (i) Y and Z, (ii) Z and X.
2. 2What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part, based on the alpha particles that bounced back or deflected sharply?
3. 3Arrange in the correct chronological order: (i) Bohr's model — fixed orbits with definite energy. (ii) Thomson's model — plum pudding. (iii) Rutherford's model — dense central nucleus. (iv) Dalton's model — indivisible particles.