Chemistry / Unit 2: Atomic Structure
2.1 History of the Atom
Notes
Our model of the atom changed as experiments gave new evidence.
- Democritus (about 400 BC) proposed tiny, indivisible particles called atoms, with no experiments.
- Dalton (1803): elements are made of atoms; atoms of one element are identical; atoms combine in whole-number ratios; atoms are rearranged, not created or destroyed, in reactions.
- Thomson (1897): cathode ray experiments found the electron. "Plum pudding" model: electrons in a ball of positive charge.
- Rutherford (1911): gold foil experiment. Most alpha particles passed through, a few bounced back, so the atom is mostly empty space with a small, dense, positive nucleus.
- Bohr (1913): electrons orbit in fixed energy levels. The modern quantum model places electrons in fuzzy regions called orbitals.
Practice
Which scientist discovered the electron using cathode rays?
Thomson showed cathode rays are made of small negative particles, later called electrons.
In Rutherford's gold foil experiment, a few alpha particles bounced almost straight back. This showed that:
Only a tiny, massive, positive center could repel alpha particles that strongly.
Most alpha particles passed straight through the gold foil. What did this suggest?
If most particles hit nothing, most of the atom's volume must be empty.
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