Planetary model
By 1911 the components of the atom had
been discovered. The atom consisted of
subatomic particles called protons and
electrons. However, it was not clear how
these protons and electrons were arranged
within the atom. J.J. Thomson suggested
the"plum pudding" model. In this model
the electrons and protons are uniformly
mixed throughout the atom:
Rutherford tested Thomson's hypothesis
by devising his "gold foil" experiment.
Rutherford reasoned that if Thomson's
model was correct then the mass of the
atom was spread out throughout the atom. Then, if he shot high velocity alpha particles (helium
nuclei) at an atom then there would be very little to deflect the alpha particles. He decided to test
this with a thin film of gold atoms. As expected, most alpha particles went right through the gold
foil but to his amazement a few alpha particles rebounded almost directly backwards.
These deflections were not consistent with Thomson's model. Rutherford was forced to discard
the Plum Pudding model and reasoned that the only way the alpha particles could be deflected
backwards was if most of the mass in an atom was concentrated in a nucleus. He thus developed
the planetary model of the atom which put all the protons in the nucleus and the electrons orbited
around the nucleus like planets around the sun.
Electric cloud model.
Erwin Schrödinger built upon the thoughts of Bohr yet took them in a new direction.
He developed the probability function for the Hydrogen atom (and a few others). The
probability function basically describes a cloud-like region where the electron is likely
to be found. It can not say with any certainty, where the electron actually is at any
point in time, yet can describe where it ought to be. Clarity through fuzziness, is one
way to describe the idea. The model based on this probability equation can best be
described as the cloud model.
The cloud model represents a sort of history of where the
electron has probably been and where it is likely to be
going. The red dot in the middle represents the nucleus
while the red dot around the outside represents an
instance of the electron. Imagine, as the electron moves
it leaves a trace of where it was. This collection of traces
quickly begins to resemble a cloud. The probable
locations of the electron predicted by Schrödinger's
equation happen to coincide with the locations specified
in Bohr's model.