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The History of
ATOMIC THEORY
The Greeks
Empedocles (490-444 BC)
The four elements: fire, water, wind, and earth.
Democritus (460 to 420 BC)
Founder of “atomism”. He believed atoms were
indivisible and indestructible.
•Greek word
“atomos” meaning
“uncut” or
“indivisible”
Democritus
Aristotle and Plato (427-322 BC)
•Said that
matter was
continuous and
could be
broken down
into atoms.
•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 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
Discovery of the Electron
Joseph John (JJ) Thomson (1856-1940)
•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
Mass and Charge of the Electron
Robert Millikan (1868-1953)
•1909 American Physicist from
University of Chicago
•Determined charge and mass
of an electron through 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
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……..
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! If the nucleus was the size of a quarter, the diameter of the
atom would be about the length of this room.
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