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).
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).