History of the Atom Democritus u Greek Philosopher

History of the Atom

Democritus u Greek Philosopher (460 -370 B. C. ) îAmong the first to suggest the existence of atoms. îBelieved that atoms were indivisible & indestructible îAgreed with later scientific theory

Democritus u His theory did not explain chemical behavior. u Lacked experimental support. u His approach was not based on the scientific method.

John Dalton u English chemist & School Teacher (1766 -1844) u 2000 years after Democritus u Used experimental methods to transform Democritus’ ideas into scientific theory.

Scientific Method u Make observations u Form a hypothesis u Test the hypothesis u Revise the hypothesis u Final products îScientific Theories îScientific Laws

John Dalton’s Atomic Theory u All elements are composed of tiny indivisible particles called atoms u Atoms of the same element are identical. u The atoms of any one element are different than those from any other element.

John Dalton’s Atomic Theory u Atoms of different elements can chemically combine in simple whole number ratios to form compounds. u Chemical reactions occur when atoms are separated, joined or rearranged. Atoms themselves are never changed into atoms of another element as a result of a chemical reaction

J. J. Thomson u English Physicist (1856 -1940). u Discovered the electron u An electron is a negatively charged subatomic particle.

J. J. Thomson Experiment u Used a cathode ray tube. u Passes an electric current through gasses at low pressure. u Results in a glowing beam called a cathode ray.

J. J. Thomson Experiment u Used (+) and (-) charged plates to affect the path of the beam. u (+) attracted the beam u (-) repelled the beam (-) plate (+) plate

J. J. Thomson Experiment u Repeated the experiment with different gasses with the same results. u Hypothesized that a cathode ray is a stream of tiny negatively charged particles which he called electrons. u Concluded that electrons must be a part of all atoms.

Robert Millikan u U. S. Physicist (1868 -1953). u Calculated the charge and mass of an electron. u 1 unit of negative charge u 1/1840 the mass of a hydrogen atom.

Protons u Since electrons are negatively charged and atoms are positively charged, atoms must contain a positive charge as well. u Electric charge must be carried on a unit of matter. u Electric charges always occur in whole number multiples of a single basic unit. u Therefore atoms must contain a particle that is positively charged.

Eugen Goldstein u German Physicist (1850 -1930). u Confirmed positively charged particles when he observed rays travelling in the opposite direction of the cathode rays in the cathode tube. u From the (+) cathode to the () anode.

Eugen Goldstein u Concluded that the ray was composed of positively charged particles. u Named them protons. u A proton has a mass of about 1840 times that of an electron.

Eugen Goldstein u Concluded that the ray was composed of positively charged particles. u Named them protons. u A proton has a mass of about 1840 times that of an electron.

James Chadwick u English Physicist (1891 -1974) u Confirmed the existence of a third sub atomic particle. u Neutron: îHas a mass similar to a proton îHas no charge