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400 B.C.

Demokritos and Leucippos use the terms "atomos"

Time of Alchemy

2000 years


Georg Bauer: systematic metallurgy
Paracelsus: medicinal application of minerals


Robert Boyle: The Skeptical Chemist. Quantitative experimentation, identification or elements


Georg Stahl: Phlogiston Theory
Joseph Priestly: Discovery of O2
Antoine Lavoisier: The role of O2 in combustion, law of conservation of mass, first modern chemistry textbook


Joseph Proust: law of definite proportion (composition)
John Dalton: the atomic theory, the law of multiple proportions
Joseph Gay-Lussac: combining volumes of gases, existence of diatomic molecules
Amadeo Avogadro: molar volumes of gases
Jons Jakob Berzelius: relative atomic masses, modern symbols for the elements
Dmitri Mendeleyev: the periodic table
J.J. Thomson: discovery of the electron
Henri Becquerel: discovery of radioactivity


Robert Millikan: charge and mass of the electron
Ernest Rutherford: existence of the nucleus, and its relative size
Meitner & Fermi: sustained nuclear fission
Ernest Lawrence: the cyclotron and trans-uranium elements

Dalton's Atomic Theory (1808)

All matter is composed of extremely small particles called atoms
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties
Atoms cannot be subdivided, created, or destroyed
Atoms of different elements combine in simple whole-number ratios to form chemical compounds
In chemical reactions, atoms are combined separated of rearranged

Modern Atomic Theory

Several changes have been made to Dalton's theory.
Dalton: atoms of a given element...and other properties
Modern #1: atoms of an element have a characteristic average madd which is unique to that element
Dalton: atoms cannot be subdivided...destroyed
Modern #2: atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions

Discovery of the Electron

1897; J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle

Cathode Ray

cathode ray tubes pass high voltage electricity through a gas that is contained at a very low pressure. When particles pass through, they slit and (-) is drawn to (+) plate, while (+) is drawn to (-) plate.
have identical properties regardless of the element (gas) used to produce them. all elements must contain identically charged electrons

Thomson's Atomic Model

J.J. Thomson believed that the electrons were like plums embedded in a positively charged "pudding" thus it was called the "plum pudding" model

Mass of the Electron

1909; Robert Millikan determines the mass of the electron with the oil drop apparatus.

Oil Drop Apparatus

see ppt

Conclusions from the Study of the Electron

cathode rays have identical properties regardless of the element used to produce them. All the elements must contain identically charged electrons
atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons
electrons have so little mass that atoms must contain other particles that account for most of the mass

Gold Foil Experiment

Rutherford: alpha particles are helium nuclei. particles were fired at a thin sheet of gold foil. particle hits on the detecting screen (film) are recorded

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