The History of The Atomic Theory

Democritus 470-380 BC Greek Philosopher Agreed with Leucippus Disciple of Leucippus Agreed with Leucippus Named small particles “atomos” (means indivisible) Thought there were different types of atoms Differed in size and shape Substances were composed of mixtures of atoms

Aristotle. ~384 BC -Opposed the idea of atomism Aristotle ~384 BC -Opposed the idea of atomism He could not believe that matter couldn’t be further separated. -He promoted the thought that all matter was composed of a mixture of earth, wind, fire, and water -Atomism was not accepted by the general public

Antoine Lavoisier 1743-1794 French Chemist Performed meticulous experiments – very accurately measured masses Discovered the Law of Conservation of Mass “Mass cannot be created or destroyed in chemical change” “Father of Modern Chemistry”

Joseph Proust 1754-1826 French Chemist Conducted careful experiments Discovered the Law of Definite Composition “A chemical compound contains the same elements in exactly the same proportion by mass regardless of the size or the source of the sample of the compound.” Ex: H2O vs. H2O2 CO2 vs. CO

John Dalton 1766 – 1844 English Chemist & School Teacher Believed that atoms were not divisible, and that atoms of each element are identical to each other. Discovered The Law of Multiple Proportions: “If two or more compounds are composed of the same elements, those elements combine in different whole number ratios (by mass) to yield different compounds.” Ex: CO vs. CO2

The Atomic Theory Authored by John Dalton in the early 1800s. Summarized the basic knowledge of the atom up until the time of Dalton. Forms our foundation for understanding the atom.

Dalton’s Atomic Theory 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 can combine in simple, whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged. **The Atomic Theory was not accepted until the late 1800s. **There are two errors with The Atomic Theory.

Crookes Tubes

JJ. Thomson (1856-1940) English Physicist Cathode Ray Tube Was able to deflect the cathode ray in an electric field (1897)_ Found the mass to charge ratio of an electron Credited with discovery of electron Plum Pudding Model

The Discovery of the Electron “Could anything at first sight seem more unpractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?” - J. J. Thomson

Robert Milliken (1868-1953) Oil Drop Experiment American Physicist Measured minimum electric charge that could be carried by a particle (1905) This enabled him to find the mass of an electron Determined it to be 1/1837 as massive as a H atom

Rutherford (1871-1953) New Zealand born physicist Performed the Au foil experiment Discovered the nucleus (1910) Discovered the proton (1918)

Rutherford (1871-1953) Discovered the nucleus (1910) Discovered the proton (1918)

James Chadwick (1891-1974) Discovered the neutron in 1932 In college he stood in the wrong line he wanted to be a Mathematician, but was in the line for physics and too embarrassed to switch Instead he became a famous physicist

Subatomic Particles Electron 9.11 x 10-31 kg -1 (-1.60 x 10-19C) Proton 1.67 x 10-27 kg +1 (+1.60 x 10-19C) Neutron 1.67 x 10-27 kg None