J.J. Thomson — discovered the electron (1897)

• Studying cathode rays (in a cathode ray tube), Thomson discovered tiny negatively charged particles — electrons.
• Electrons are found in every element → atoms are NOT indivisible after all!
• Charge of an electron = –1.602 × 10⁻¹⁹ C, taken as –1 by convention.

Cathode ray tube: rays flow from cathode (–) to anode (+)

Puzzle: atoms are neutral — so where’s the positive charge? Thomson proposed:

“Plum pudding” model: electrons embedded in a positive sphere

Like a watermelon: red pulp = positive matter, seeds = electrons

In 1911, Geiger & Marsden (under Rutherford) fired α-particles (positively charged, from radioactive elements) at a super-thin gold foil to test Thomson’s model.

Most particles pass straight through — but a few scatter sharply, and some bounce back!

• Expected (per Thomson’s model): particles pass straight through or deflect only slightly.
• Observed: most passed through undeflected, some deflected sharply, and a few bounced straight back!
• This deflection from the straight path is called scattering — hence “α-ray scattering experiment.”
• Thomson’s model failed to explain this result.

Ernest Rutherford — Father of Nuclear Physics

Planetary model: electrons orbit a tiny, dense nucleus

• Most of the atom is empty space — since most α-particles passed straight through.
• The nucleus is tiny, dense, and holds all the positive charge + most of the mass.
• Electrons revolve around the nucleus like planets around the Sun — the planetary model.
• Nucleus is about 10⁵ times smaller than the atom (atom ≈ 10⁻¹⁰ m, nucleus ≈ 10⁻¹⁵ m).

Limitation: Rutherford’s model couldn’t explain why atoms are stable. A revolving (accelerating) electron should lose energy and spiral into the nucleus — but atoms don’t collapse!

Rutherford’s model predicted electrons would spiral in and collapse — but they don’t!

Niels Bohr — proposed fixed energy levels (1913)

Shells K, L, M, N… — energy increases moving outward

• Electrons move only in fixed circular paths called stationary states / orbits / shells — not randomly.
• Shells named K, L, M, N… (or n = 1, 2, 3, 4…). K is closest to the nucleus and has the least energy.
• While in a fixed shell, an electron does NOT lose energy — this is what explains atomic stability!
• An electron can jump shells only by absorbing or releasing a fixed amount of energy.

James Chadwick discovered the neutron (1932)

Puzzle: Helium has 2 protons but its mass is ~4× that of hydrogen (1 proton), not 2×. Something else must add mass!

• James Chadwick discovered the neutron — mass ≈ proton’s mass, but no charge (symbol: n).
• Found in the nucleus of all atoms except hydrogen.
• Neutrons help hold the nucleus together by weakening the repulsion between protons — via the nuclear force.

BARC, Mumbai — India’s centre for neutron-scattering research

Dalton’s original pictorial symbols (1803) — later replaced by letters

• Dalton (1803): first introduced pictorial symbols for elements.
• Berzelius (1813): proposed symbols from elements’ Latin names — the alphabetic system we use today.
• Today, IUPAC approves official names & symbols worldwide.

Rules for writing symbols:

• First letter is always capital; second letter (if any) is small — e.g. Al (not AL), Co (not CO).
• Some symbols use a letter other than the 2nd letter — e.g. Chlorine = Cl, Zinc = Zn.
• Some symbols come from Latin/Greek/German names — Iron = Fe (ferrum), Mercury = Hg (hydrargyros), Tungsten = W (wolfram).

Lithium: 3 protons, 4 neutrons, 3 electrons

• Electron mass is negligible — mass of an atom comes almost entirely from protons + neutrons.
• Standard notation: mass number written top-left, atomic number bottom-left of the symbol.

Bohr-Bury rules for filling electrons into shells:

• Maximum electrons in a shell = 2n² (n = shell number): K=2, L=8, M=18…
• Outermost shell can hold a maximum of 8 electrons (except the very first shell, which holds max 2).
• Shells fill in order — K first, then L, then M… — the next shell only starts filling once the current one is full.

Electron shell diagrams for the first 18 elements (H to Ar)

This electron arrangement is called the electronic configuration of an atom (e.g. Sodium = 2, 8, 1).

Hydrogen’s isotopes: protium, deuterium, tritium (1, 2, 3 nucleons)

Carbon’s isotopes: C-12, C-13, C-14 (same protons, different neutrons)

• Isotopes have the same chemical properties (same electrons/valence), but different physical properties (boiling/melting points).
• Uses: U-235 → nuclear fuel; Co-60 → cancer radiation treatment; I-131 → thyroid treatment; C-14 → dating fossils.

Uranium-235 fuels nuclear power plants

e.g. Chlorine: 75% ³⁵Cl + 25% ³⁷Cl → weighted average = 35.5 u (not the simple average of 36 u!).

Homi Bhabha — father of India’s nuclear programme

• Example: Calcium (Z=20), Potassium (Z=19), Argon (Z=18) — all have mass number 40, but are different elements.

STM image — individual atoms made visible!

The atomic story isn’t over — Bohr’s model too had limits. Electrons don’t follow neat fixed paths; today we picture them as “electron clouds” — regions of probability around the nucleus (the quantum mechanical model, studied in higher grades).