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Rutherford Institute Chemistry · Intro

The Story of
the Atom

From Ancient India & Greece
to Quarks and Gluons

Learn Chemistry Through Logic and Visualization

@RutherfordInstituteChemistry

Class 9–10 Science · Class 11–12 Chemistry · NEET · CBSE

The big question

What is everything made of?

Matter
a drop ~ 1 mm
Molecule
H₂O ~ 0.3 nm
Atom
~ 0.1 nm
Nucleus
~ 10⁻¹⁴ m
Proton
~ 10⁻¹⁵ m

Keep zooming in… what do we find at the very bottom?

Rutherford Institute Chemistry · The Story of the Atom
Ancient India & Ancient Greece

Two ancient ideas of the atom

India · c. 6th–2nd century BCE (dates uncertain)

Acharya Kanada

  • Founder of the Vaisheshika school
  • Paramanu (परमाणु): tiny, eternal, cannot be divided
Greece · c. 5th century BCE

Democritus

  • Atomos = uncuttable
  • With teacher Leucippus: keep cutting gold until it cannot be cut
Ideas from pure reasoning, no experiments yet
Rutherford Institute Chemistry · The Story of the Atom
1803 – 1808 · John Dalton

Dalton’s atomic theory

  • All matter is made of tiny atoms
  • Atoms can’t be made, destroyed or split in a reaction
  • Atoms of one element are identical in mass and properties
  • Atoms combine in small whole-number ratios

The “billiard ball” model

Rutherford Institute Chemistry · The Story of the Atom
1897 · J. J. Thomson

Discovery of the electron

Thomson cathode-ray tube apparatus (static illustration)
Thomson’s cathode-ray tube
  • Cathode rays bend towards the + plate → they are negative
  • e/m ≈ 1.76 × 1011 C/kg
  • Same for every metal → inside every atom
  • About 1/1836 the mass of a hydrogen atom
Rutherford Institute Chemistry · The Story of the Atom
1904 · Thomson’s model

The plum pudding model

  • A ball of positive charge
  • Electrons sit inside it, like plums in a pudding
  • Total charge: zero (neutral)
  • NCERT compares it to a watermelon with seeds

Positive “pudding” · negative “plums”

Rutherford Institute Chemistry · The Story of the Atom
1909 · Geiger & Marsden, in Rutherford’s lab

The gold foil experiment

α source in lead box gold foil screen flashes where α particles hit

Expected · plum-pudding atom

all pass straight through

Observed · nuclear atom

most pass · some deflect · a few bounce back!

Rutherford Institute Chemistry · The Story of the Atom
Interactive · 3D

Live · Rutherford gold-foil experiment

Try it: switch Thomson vs Rutherford and watch the flashes.

Loading 3D experiment…
Rutherford remembers · lecture, 1936

Quite the most incredible event…

It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.

— Ernest Rutherford

Only something tiny, heavy and positive could throw a fast alpha particle back.
1911 · Rutherford’s nuclear model

A tiny, dense nucleus

  • Nucleus: tiny, positive, holds almost all the mass
  • Electrons move around it
  • The atom is mostly empty space
Cricket stadium analogy: atom as stadium, nucleus as marble
atom = whole stadium · electrons in the stands nucleus = marble
Rutherford Institute Chemistry · The Story of the Atom
1964 – 1968 · Quarks

Protons and neutrons are made of quarks

Proton = u u d
+⅔ +⅔ −⅓ = +1
Neutron = u d d
+⅔ −⅓ −⅓ = 0
  • Idea: Gell-Mann and Zweig, 1964
  • Quarks have fractional charges, +⅔ and −⅓
1909 · α particles + gold atom Found: the nucleus
1968 · fast electrons + proton (SLAC) Found: quarks!
SLAC, 1968: a Rutherford-style scattering experiment, using electrons instead of alphas!
Rutherford Institute Chemistry · The Story of the Atom
The journey so far

2,500 years of atomic models

Ancient Kanada & Democritus indivisible particles
1808 Dalton solid sphere
1904 Thomson plum pudding
1911 Rutherford nuclear atom
1913 Bohr energy levels
1926 Schrödinger electron cloud
1964–79 Quarks & gluons inside the proton

Each new experiment changed the picture: that’s how science works!

Rutherford Institute Chemistry · The Story of the Atom
For Class 9–12 · NCERT · CBSE · NEET

Why this matters for you

See it

Picture every model in your mind: no blind memorising

Class 9: Structure of the Atom

Understand it

Know WHY each model changed: experiment → new idea

Class 11: Structure of Atom

Score it

NEET and CBSE love concept questions on atomic models

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Rutherford Institute Chemistry · The Story of the Atom