Early Atomic Theory

Empedocles (492-432 BC)
Proposed all matter was made of 4 elements: fire, water, air and earth. Matter had different ratios of these 4 elements.

Democritus (460-370 BC)

• theory based on reasoning 
• smallest matter called ATOMOS
• ATOMOS = indivisible
• Atomos can't be destroyed
• Atoms of given material are the same

Aristotle & Plato
Contemporaries of Democritus debunked his theory in support of Empedocles & added the 4 elements could be transformed into each other.

TORICELLI (1643)
invented the barometer and showed air had weight

DANIEL BERNOULLI
His theory -- air & gases are collections of ball-like particles that move and collide with each other

ROBERT BOYLE (1660)
Developed new definition of element -- fundamental substances that can't be broken down further by chemical means

JOSEPH PRIESTLY (1773)
Through experimentation found substances could combine or break apart into new substances with new properties.

Antoine Lavoisier (1778)
Experimented with Oxygen (deplogiston) and Hydrogen (phlogiston) and formed water. Proved Law of Conservation of Mass

Law of Definite Proportions
States that a chemical compound always contains the same proportion of elements by mass.

Law of Multiple Proportions
When chemical elements combine they do so in a ratio of small whole numbers: C and O combine to form CO and CO2

John Dalton (1808) proposed Atomic Theory

• All matter composed of indivisible particles called atoms
• All atoms of given element are identical
• Atoms cannot be subdivided, created or destroyed
• Chemical reactions combine, separate or rearange atoms
• Elements form compounds in defined whole-number ratios

JJ Thomson (1897)
From the Cathode Ray tube experiment concluded the rays were negatively charged particles (electrons) and measured the charge to mass ratio of the ray particles

Plum Pudding Model of the Atom

Robert Millikan (1909)
From his famous oil drop experiment the mass of an electron was calculated.

E. Rutherford
Gold Foil Experiment showed the atom was mostly empty space and the atom has a positively charged center that contains most of its mass

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