Atomic Structure

Syllabus Unit: Physical Chemistry

Why it matters

Chapter overview

Atomic structure traces the ~50-year dismantling of Dalton's 'indivisible atom' — from Thomson's electron and Rutherford's nucleus to Bohr's quantized orbits and finally the full quantum-mechanical picture of orbitals and quantum numbers. JEE builds every periodic-trend and bonding question on this model, so the postulates, formulas (En, rn) and quantum number rules here are non-negotiable foundations.

Chemistry Syllabus Structure Physical Chemistry ~33% weightage

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Step 11886 — GoldsteinCanal rays reveal the proton

Step 21897 — ThomsonCathode rays reveal the electron

Step 31909 — MillikanOil drop fixes electron charge

Step 41932 — ChadwickBeryllium bombardment reveals the neutron

1Discovery of Electron, Proton & Neutron

Three experiments across 46 years replaced Dalton's solid, indivisible atom with a structure of charged, weighable parts.

Learning Objectives

Describe Thomson's cathode ray tube experiment and what it revealed about the electron.
Describe Millikan's oil drop experiment and its role in fixing the electron's exact charge.
Describe Goldstein's canal ray experiment that led to the discovery of the proton.
Describe Chadwick's experiment that discovered the neutron and explain why it was needed.

Key Terms

Cathode RaysStreams of electrons emitted from a discharge tube's cathode, identical regardless of the gas or electrode material used.

e/m Ratio1.7588 × 10¹¹ C/kg — the electron's charge-to-mass ratio, measured by Thomson from cathode ray deflection.

Oil Drop ExperimentMillikan's method that found electric charge always comes in integer multiples of 1.602 × 10⁻¹⁹ C.

Canal RaysPositively charged particles discovered by Goldstein moving opposite to cathode rays; the lightest, from hydrogen gas, was the proton.

Between 1886 and 1932, four experiments dismantled Dalton's idea of a solid, indivisible atom, replacing it piece by piece with electron, proton and neutron.

Electron: J.J. Thomson (1897)

In a cathode ray discharge tube, rays travelling from cathode to anode bent toward a positive plate — proving they carried negative charge.
Thomson measured e/m = 1.7588 × 10¹¹ C/kg, and found this value identical regardless of the gas or electrode material — the electron is a universal subatomic particle.

Fixing the electron's charge: Millikan (1909)

Millikan's oil drop experiment balanced gravity against an applied electric field on charged oil droplets, finding that charge always appeared as an integer multiple of **1.602 × 10⁻¹⁹ C**. Combined with Thomson's e/m, this gave the electron's mass: 9.1 × 10⁻³¹ kg.

Proton and neutron

Goldstein's canal rays (1886) — positively charged rays moving opposite to cathode rays; their mass depended on the gas used, and the lightest (from hydrogen) was the proton, mass 1.6726 × 10⁻²⁷ kg.
Chadwick (1932) bombarded beryllium with alpha particles, producing a neutral, highly penetrating radiation — the neutron, mass close to the proton's, confirming the 'missing' mass in the nucleus.

Key PointThe neutron's discovery explained isotopes — atoms of the same element with the same proton count but different neutron counts, hence different mass numbers.