The History of ATOMIC THEORY As told by Ms. Hummel

The Greeks: Empedocles 490-444 BC Leucippus and Democritus 460-420 BC Plato 427-347 BC Aristotle 384-322 BC

Empedocles (490-444 BC)

Leucippus and Democritus (460 to 420 BC) Founders of “atomism” Democritus

Aristotle and Plato (427-322 BC) Said that matter was continuous and could be broken down Greek word “atomos” meaning “uncut” or “indivisible” ATOM: smallest particle of an element that retains the chemical identity of the element

…and for the next 2000 years atomic theory was based on abstract thinking, not experimentation until . . .

John Dalton (1766-1844) English school teacher Work completed 1803-1807 Presented Dalton’s Atomic Theory

Dalton’s Atomic Theory 1. All elements are composed of submicroscopic, indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one elements are different from those of any other element. 3. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. 4. Chemical reactions occur when atoms are separated, joined or rearranged. However, atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

Ben Franklin (1706-1790) Proved electricity existed

Michael Faraday (1794-1867) Suggested atomic structure was related to electricity

Discovery of the Electron Joseph John (JJ) Thomson (1856-1940)

Constructed cathode ray tube Found rays bent towards a + charge

Concluded rays were negatively charged particles Called the particles “electrons” after the Greek word “elektron” meaning amber

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen which itself is so small that a crowd of these atoms equal in number to the population of the whole world would be too small to have been detected by any means then known to science” -- J.J. Thomson.

Mass and Charge of the Electron Robert Millikan (1868-1953) 1909 American Physicist from University of Chicago Determined charge and mass of an electron though the famous “oil drop” experiment

Charge of an electron: 1.60 x 10 -19 coulomb Mass of an electron: 9.11 x 10-28 g

ONWARD TO THE PROTON

History of discovery of the proton is rather vague: Eugene Goldstein (1886) - Used cathode ray tube but with Ne atoms with electrons removed Called “positive rays” because they went from positive electrode to negative J.J. Thomson (1906) Studied positive ray deflection When hydrogen was used, he got the smallest particles Called the particles PROTONS

Notice that the rays are emitted from the POSITIVE electrode ( positive rays!)

Neutral atoms have an equal number of electrons and protons Mass of protons are 1836 time greater than the mass of electrons So…….. How are the electrons and protons arranged in an atom? We had our theories……..

“cookie dough” or “plum pudding” model J.J. Thomson “cookie dough” or “plum pudding” model Protons and electrons embedded together in atom like chocolate chips in cookie dough!

1903 Nagaoka “Saturnian” model of atom with flat rings of electrons revolving around a positively charged particle

1909 Ernest Rutherford conducted the famous “gold foil” experiment BUT THEN….. 1909 Ernest Rutherford conducted the famous “gold foil” experiment

This led to a new model for the atom…….. A dense nucleus of positive charge with the electrons circling around it Size scale: if the nucleus of the atom was the size of a tennis ball, the atom would have a diameter over 1 mile. The nearest electron would be .25 mi from the nucleus!

So this is where we’ve come so far…..

Neutral atoms have an equal number of electrons and protons Mass of protons are 1836 time greater than the mass of electrons Did not explain why the mass of He was 4 times the mass of hydrogen ONWARD TO THE NEUTRON

James Chadwick (1932)

But that’s not all, folks. In 1922 a gentleman by the name of Niels Bohr developed an explanation of atomic structure that underlies the regularities of the periodic table.

His atomic model had atoms built of successive orbital SHELLS of atoms

And then, of course, in 1930, Schrodinger viewed electrons as continuous clouds and introduced “wave mechanics” as a mathematical model of the atom The mathematical equation is: …but we’ll talk about Bohr and Schrodinger later!

………and this is only the beginning