Background
Solid Sphere Model
John Dalton proposed that all matter is made of indivisible atoms that combine in fixed whole-number ratios to form compounds.
Plum Pudding Model
J.J. Thomson discovered the electron using cathode ray tubes, demonstrating atoms have internal subatomic structure with negative charges in a positive sphere.
Nuclear Model (Gold Foil Experiment)
Alpha particle scattering proved the atom has a tiny, dense, positively charged nucleus containing virtually all the atom's mass, surrounded by mostly empty space.
Planetary Shell Model
Niels Bohr demonstrated that electrons occupy quantised circular energy levels (shells), explaining the discrete line emission spectrum of hydrogen.
Why Carbon-12 is the Unified Standard
Historically, chemists and physicists used different isotopes of Oxygen, causing data confusion. Carbon-12 was chosen internationally in 1961 because:
- It is a solid that is easily handled and stored.
- It is readily available and extraordinarily abundant in organic and inorganic chemistry.
- It gives atomic weights very close to whole numbers for most common elements.
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