Journey Inside the Atom – Class 9 Science Chapter 8 Notes & NCERT PDF

Think It Over

Introduction

Everything around us — our bodies and the bricks of a house — is ultimately built from atoms, far too small to see. This chapter follows the journey of ideas about the atom, from ancient thinkers to modern models, and shows how science moves forward one experiment at a time.

Key Concepts

8.1 Roots of atomic theory

8.2 Atomic models

Discovery of the electron (1897) — J. J. Thomson passed electricity through gases at low pressure in a cathode ray tube. The cathode rays were streams of negatively charged particles much lighter than atoms — electrons — and they were the same whatever the gas or cathode material. Charge of an electron: −1.602 × 10⁻¹⁹ C, taken as −1. (Nobel Prize 1906.)

Thomson's model — a sphere of positive charge with electrons spread through it: the plum pudding model (the book compares it to a watermelon: red pulp = positive charge, seeds = electrons).

Gold foil experiment (1911) — Geiger and Marsden, under Rutherford, fired alpha particles (positive; helium nuclei with 2 protons and 2 neutrons) at a very thin gold foil. Most passed straight through, some were deflected sharply, a few bounced back. Thomson's model could not explain this.

Rutherford's (planetary) model

Limitation — an electron moving in a circle is accelerating and should lose energy, spiral in and crash into the nucleus. Atoms would collapse — but they don't.

Protons — Rutherford showed the nuclear positive charge comes from protons, much heavier than electrons, with equal and opposite charge. A neutral atom has equal numbers of protons and electrons (helium 2 and 2; sodium 11 and 11).

Bohr's model (1913) — Niels Bohr proposed:

8.3 The neutron (1932)

Helium has 2 protons but about 4 times the mass of hydrogen. James Chadwick discovered the neutron — mass about equal to a proton, no charge (Nobel Prize 1935). All nuclei except ordinary hydrogen contain neutrons. Heavier atoms need extra neutrons to reduce proton repulsion (iron: 26 protons, 30 neutrons; uranium: 92 protons, 146 neutrons). BARC's Dhruva reactor is used for neutron-scattering research in India.

Particles: electron e⁻ (−1), proton p⁺ (+1), neutron n⁰ (0).

8.4 Symbols of elements

Dalton (1803) used pictorial symbols; Berzelius (1813) used letters from Latin names; today IUPAC approves names and symbols. First letter capital, second small: Al, Co (not CO), Cl, Zn. Latin/other roots: Fe (ferrum), Cu (cuprum), Ag (argentum), Au (aurum), Na (natrium), K (kalium), Pb (plumbum), Hg (hydrargyros), W (wolfram). About 118 elements are known.

8.5–8.6 Atomic number and mass number

Atomic number (Z) = number of protons — it identifies the element.

Mass number (A) = protons + neutrons (nucleons).

Written as ᴬ_Z X, e.g. carbon ¹²₆C.

8.7 Distribution of electrons (Bohr–Bury rules)

8.8 Valency

The valence shell is the outermost shell; its electrons are valence electrons. Eight electrons (two for helium) make a stable octet. Valency = electrons lost, gained or shared to complete the octet.

8.9 Isotopes and isobars

Isotopes — same atomic number, different mass number (different neutrons). Hydrogen: protium ¹H (~99.98%), deuterium ²H (~0.015%), tritium ³H (traces). Carbon: C-12 (most abundant), C-13, C-14. Isotopes have the same chemical properties (same electrons) but different physical properties.

Uses: uranium-235 as nuclear fuel, cobalt-60 in cancer radiation therapy, iodine-131 for goitre and thyroid cancer, carbon-14 for dating fossils and artefacts.

Average atomic mass — chlorine is 75% ³⁵Cl and 25% ³⁷Cl, so the weighted average is 35 × 0.75 + 37 × 0.25 = 35.5 u (the simple mean 36 u would be wrong).

Isobars — different elements with the same mass number: argon (18), potassium (19), calcium (20) all have A = 40.

Homi Jehangir Bhabha, father of India's nuclear programme, founded TIFR and BARC.

Worked Numericals

Activities and Explorations

Key Terms

Common Misconceptions

Exercise Hints