Download Summary: History of Models of the Atom

Atomic Models Review Sheet
December 2, 2012
Democritus (460-370 BC) First proposed the existence of an ultimate particle, a particle that could not
be divided. He used the word "atomos" to describe this particle. He thought that there were different
types of atoms with specific sets of properties.
Aristotle (384-322 BC) believed in the four elements of air, earth, water and fire. Aristotle felt that
regardless of the number of times you cut a form of matter in half, you would always have a smaller
piece of that matter. This view held sway for 2000 years primarily because Aristotle was the tutor of
Alexander the Great.
John Dalton (1776-1844) proposed the theory that all matter is made up of individual particles called
atoms, which cannot be divided. The main points of Dalton’s theory are as follows:
•All elements are composed of atoms
•All atoms of the same element have the same mass, and atoms of different elements have
different masses
•Compounds contain atoms of more than one element
•In a particular compound, atoms of different elements always combine in the same way.
J. J. Thomson (1856-1940) identified the negatively charged electron in a cathode ray tube
experiment in 1897. He deduced that the electron was a component of all matter. Thomson
proposed the "plum pudding" model of the atom. In this model, the volume of the atom is composed
primarily of the more massive (thus larger) positive portion (the pudding). The smaller electrons
(actually, raisins in the plum pudding ) are dispersed throughout the positive mass to maintain charge
neutrality.
Nagaoka suggested a "Saturnian" model of the atom with flat rings of electrons revolving around a
positively charged particle.
Ernst Rutherford (1871-1937) proposed the nuclear atom as the result of the gold-foil experiment in
1911. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a
small, dense volume at the center of the atom and that most of the volume of the atom was empty
space occupied by the electrons. The electrons moved randomly in the space around the nucleus.
Although positive particles had been discussed for some time, it was Rutherford in 1920 that first
referred to the hydrogen nucleus as a proton. Also in 1920, Rutherford proposed the existence of
the third atomic particle, the neutron.
Schrodinger viewed electrons as continuous clouds and introduced "wave mechanics" as a
mathematical model of the atom.
James Chadwick (1891-1974) discovered the neutron in 1932. Chadwick was a collaborator of
Rutherford's. Interestingly, the discovery of the neutron led directly to the discovery of fission and
ultimately to the atomic bomb.
Bohr developed an explanation of atomic structure that underlies the regularities of the periodic
table of elements. His atomic model had atoms built up of successive orbital shells of electrons. The
electrons moved in spherical orbits at fixed distances from the nucleus.
The electron cloud model is an atom model wherein electrons are no longer depicted as particles
moving around the nucleus in a fixed orbit. Instead, we shouldn’t know exactly where they are, and
therefore describe their probable location around the nucleus only as an arbitrary ‘cloud’. It has been
found that, contrary to Rutherford’s popular model, we can only describe electron locations in terms
of most probable regions with fuzzy boundaries – thus the term ‘cloud’. The probability of finding an
electron is greater in the denser region of the cloud. The term “electron cloud” was used by Richard
Feynman, an American physicist.