History of Atomic Theory
It all started with the ancient Greeks trying to explain the nature of matter.
Democritus
- Various basic elements which made up matter
- Matter composed of small atoms moving in a void
- Properties
- Round, pointy, oily, have hooks, etc.
- Ideas were discredited because a void meant no existence (Aristotle/Plato)
- Void = atomos
- Atom = smallest indivisible unit
- Limitation with this theory, matter can’t change.
Opposing View – Aristotle/Plato
- Earth, Air, Fire, Water were the four elements
Middle Ages Renaissance
- Alchemists techniques equipment Practical knowledge no theory
- “Lead into gold”
Robert Boyle = 1627-1691
- Substance was an element unless it could be broken down to two or more simpler substances
- What is wrong Aristotle and Plato’s four elements?
- Air =
- Water =
- Fire =
- Earth =
Foundations of Dalton’s Atomic Theory
- Lavoisier (1790’s)
- Law of conservation of mass = mass is neither created nor destroyed only changed.
- Joseph Proust 1754-1826
- Law of Definite Proportions = A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.
- A compound has fixed ratios of masses of elements (H2O)
- Dalton
- Law of Multiple Proportions = different compounds of the same elements have mass of those elements proportional to small whole numbers
- Different compounds of same elements have mass ratios of elements in small, whole numbers – CO and CO2
Modern Atomic Theory
- Four postulates of John Dalton (1766-1844)
- Postulate #1 = Each element is composed of extremely small particles called atoms
- Postulate #2 = All atoms of any given element are identical; the atoms of different elements are different and have different properties (including mass)
- Postulate #3 = Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed.
- Postulate #4 = Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms.
J.J. Thompson (1856-1940)
- Discovered electron
- 1897 – Cathode Ray Experiment
- Plum Pudding model 1904
- Electrons in a soup of positive charges
- Discovered isotopes in 1913
Cathode Ray Tube Experiment
Plum Pudding Model
- Subatomic Particles
- Same for all elements
Ernest Rutherford (1871-1937)
- Gold Foil Experiment
Rutherford Model
- Electrons – not much mass, mostly empty space around nucleus
- Dense, positive nucleus