IB CHEMISTRY

1.2

History of the Atom

Background

1803

John Dalton

Solid Sphere Model. Atoms are indivisible.

Plum

J.J. Thomson (1897)

Discovered Electron. Plum Pudding Model.

R

Rutherford (1909)

Gold Foil Exp: Discovered dense, positive Nucleus. Atom is mostly empty space.

B

Niels Bohr (1913)

Electrons orbit in fixed energy levels (shells).

Deep Think Concept

Why is Carbon-12 the standard?

Historically, chemists and physicists used different isotopes of Oxygen, causing data confusion. Carbon-12 was chosen in 1961 because:

  • It is a solid at room temperature (easy to handle in Mass Spec).
  • It is highly abundant (~98.9%).
  • It results in other elements having masses close to whole numbers (convenient for calculations).
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