The Changing Atomic Model
Name
Year
Democritus
(Greek)
400 BC
John Dalton
(English)
1803
Joseph Thomson
(English)
1904
Description
D – material could be broken down to a point where it can’t be
broken down anymore. These uncuttable “atomos” are the building
blocks of all things. Aristotle told everyone Democritus was wrong.
JD – Supported Democritus’ ideas. Had technology to back him up.
Kept the name “atom.” Determined that all matter was made up these
atoms coming together.
Came up with the idea of negatively charged “electrons” that were
stuck in a positively charged mass. Think of chocolate chips in cookie
dough. Chips are negative, dough is positively charged mass. (He
originally said raisins in bread pudding.)
1911
Ernest Rutherford
(English)
Niels Bohr
(Danish)
Shot a thin sheet of gold foil with particles and found that most went
right through. This told him that most of the atom was empty space.
Decided that the mass was concentrated in a positively charged center
(nucleus) with electrons outside.
1913
Guessed that electrons orbited around the nucleus in set paths, based
on energy levels. Lower energy electrons were in closer orbits.
Also included in this idea is the discovery of neutrons (discovered by
James Chadwick who was a student of Rutherford’s).
Electron Cloud
Model
(predicted by Erwin
Schrödinger,
supported by
Werner Heisenberg)
1990s
Electrons don’t travel in fixed orbits. However, there are areas that
they are more likely to appear in. Their locations can be connected to
their energy levels.
(1933,
1927)
Heisenberg Uncertainty Principle: At any moment you can tell the
speed of the electron or the location, but not both at the same time.
Picture