The History of

ATOMIC THEORY

The GreeksEmpedocles (490-444 BC)

The four elements: fire, water, wind, and earth.

Democritus (460 to 420 BC)

Democritus

Founder of “atomism”. He believed atoms were indivisible and indestructible.

•Greek word “atomos” meaning “uncut” or “indivisible”

Aristotle and Plato (427-322 BC)

•ATOM: smallest particle of an element that retains the chemical identity of the element

•Said that matter was continuous and could be broken down into atoms.

…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 element are different from those of any other element.

3. Atoms of different elements can physically mix together to form a mixture. Or they 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

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

•Constructed cathode ray tube•Found rays bent towards a + charge and bent away from a negative plate.

•Concluded rays were negatively charged particles

•Called the particles “electrons” after the Greek word “elektron” meaning amber

Robert Millikan (1868-1953)

•Determined charge and mass of an electron through the famous “oil drop” experiment

•1909 American Physicist from University of Chicago

Mass and Charge of the Electron

•Charge of an electron: 1.60 x 10 -19 coulomb

•Mass of an electron: 9.11 x 10-28 g

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

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

BUT THEN…..

1909 Ernest Rutherford conducted the famous “gold foil” experiment

This led to a new model for the atom……..

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! If the nucleus was the size of a quarter, the diameter of the atom would be about the length of this room.

A dense nucleus of positive charge with the electrons circling around it

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 (about 2000 times) 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 CONTINIOUS 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