Early Modern Atomic Theory History

Aristotle’s theory- all matter can be grouped into four main elements, fire, water, air, and earth

• matter can be part of multiple elements. EX: a Rabbit is air, water, and fire (Life)

Democritus’s idea- all matter is made up of tiny, indestructible particles called atoms

• he didn’t have any experimental evidence though, so people continued to believe Aristotle instead

Alchemy- both a philosophy and a hunt for ways to turn lead (or another common material) into gold so the alchemists could be closer to God.

• although their philosophy was wrong, they developed and used the scientific method and a lot of the lab techniques that we still use today

Law of conservation of matter- matter is neither created nor destroyed

Law of definite proportions- a given chemical compound always contains its component elements in fixed ratio and does not depend on its source and method of preparation.

John Dalton: had five main theories

1. matter is composed of tiny particles called atoms
2. atoms are fundamental, indestructible, and indivisible particles
3. all atoms of a given atom are exactly alike
4. atoms of one element cannot change into another element, but they can combine in simple whole ratios to make compounds
5. In a chemical reaction, atoms are separated, combined, or rearranged

Theory 2 is partially incorrect because Atoms actually are divisible. Theory 3 is incorrect because there are different isotopes and ions of atoms that have small differences between each other. (See my note “isotopes and ions” for more)

JJ Thomson- discovered the electron using the catholic ray tube

• cathode ray tube: electricity passing through a near vacuum
• showed that the radiation was a negatively charged particle smaller than an atom

Millikan: used oil drop experiment to measure the charge and mass of a single electron

Rutherford: used the gold foil experiment to discover the nucleus

• launched particles through a sheet of gold. Most went through, but some occasionally bounced back because it hit the nucleus

Chadwick: found the neutron