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Period: 500 BCE to 300 BCE
Democritus
atomist; matter could be broken down into minute size pieces called atoms; solids had rough atoms, and liquids had smooth atoms; atoms were constantly in motion -
Period: 384 BCE to 322 BCE
Aristotle
expanded on the idea of the four elements of matter (fire, earth, water, and air) and included properties (hot, cold, wet, and dry) -
Period: to
John Dalton
In an effort to explain the law of conservation of mass, he made an atomic theory:
1. All matter is composed of extremely small particles called atoms.
2. Atoms of a given element are identical in size, mass, and other properties.
3. Atoms cannot be subdivided, created, or destroyed.
4. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined, separated, or rearranged. -
Period: to
Sir William Crookes
discovered that cathode rays have mass and appear to be negatively charged -
Period: to
Joseph John Thomson
performed experiments with cathode-ray tubes and discovered that ever cathode ray has identical negatively charged particles that he named electrons; every electron has a large charge-to-mass ratio -
Period: to
Robert A. Millikan
measured the charge of the electron, which let to scientists using that information to conclude that the mass of the electron is about one two-thousandth of the mass of the simplest hydrogen atom -
Period: to
Zealander Ernest Rutherford
conducted an experiment in which a thin piece of gold foil was bombarded with alpha particles, which have a positive charge and are four ties the mass of a hydrogen atom; the alpha particles scattered, which led Rutherford to the conclusion that the majority of the atoms' masses and positive charge is concentrated in a small space in the center, which he called the nucleus