Atomic models

Of course, the Greeks started it all…. Around 440 BC, Leucippus of Miletus originated the atom concept. He and his pupil, Democritus (c460-371 BC) of Abdera, refined and extended it in future years. There are five major points to their atomic idea.

Their points: Matter is composed of atoms, which are too small to be seen and cannot be split into smaller particles. There is a void; an empty space between atoms. Atoms are completely solid. Atoms are homogeneous (the same throughout) Atoms come in all sorts of shapes and sizes.

Aristotle was against the atomists! Aristotle believed in the earth, air, fire water theory.

Not much happened in terms of atomic theory for a long time….. ….everyone was too busy engaging in bloody wars, witch burnings or dying in horrible plagues etc.

Dalton’s atom (1766-1844): 1) chemical elements are made of atoms 2) the atoms of an element are identical in their masses 3) atoms of different elements have different masses 4) atoms only combine in small, whole number ratios such as 1:1, 1:2, 2:3 and so on.

The electron On April 30, 1897, Joseph John (J.J.) Thomson (1856-1940) announced that cathode rays were negatively charged particles which he called 'corpuscles.' He also announced that they had a mass about 1000 times smaller than a hydrogen atom

Thomson’s experiments

Thomson’s atom

Rutherford’s experiment

Rutherford experiment animation http://www.waowen.screaming.net/revision/nuclear/rsanim.htm

They tried other metals as well.. For platinum, Geiger and Marsden reported: "Three different determinations showed that of the incident a-particles about 1 in 8000 was reflected, under the described conditions." There were three major findings: 1) Almost all of the alpha particles went through the gold foil as if it were not even there. Those alpha particles, of course, continued on a straight-line path until they hit the detector screen. 2) Some of the alpha particles were deflected only slightly, usually 2° or less. Geiger found that an alpha particle was, on average, deflected about 1/200th of a degree by each single encounter with a gold atom. The most probable angle of deflection for one gold foil turned out to be about 1°. (Rutherford cites a figure of 0.87° in his 1911 paper.) 3) A very, very few (1 in 8000 for platinum foil) alpha particles were turned through an angle of 90° or more. (Rutherford cites 1 in 20,000 for gold in his 1911 paper.)

Rutherford’s atom

Bohr "An expert is a man who has made all the mistakes which can be made, in a very narrow field."

Bohr (1885 - 1962 ) Ernest Rutherford had recently suggested the atom had a miniature, dense nucleus surrounded by a cloud of nearly weightless electrons. There were a few problems with the model, however. For example, according to classical physics, the electrons orbiting the nucleus should lose energy until they spiral down into the center, collapsing the atom. Bohr proposed adding to the model the new idea of quanta put forth by Max Planck in 1901. That way, electrons existed at set levels of energy, that is, at fixed distances from the nucleus. If the atom absorbed energy, the electron jumped to a level further from the nucleus; if it radiated energy, it fell to a level closer to the nucleus.

Bohr

Bohr model interactive…hours of fun http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/59229/Bohr_Nav.swf::The%20Bohr%20Atom

Chadwick’s atom Intrigued by Rutherford's speculation about a subatomic particle with no charge, Chadwick began a series of experiments to demonstrate the existence of such a particle. Initially, none of the experiments succeeded. Then, in 1930, Walther Bothe and Herbert Becker described an unusual type of gamma ray produced by bombarding the metal beryllium with alpha particles. Chadwick recognized that the properties of this radiation were more consistent with what would be expected from Rutherford's neutral particle. The subsequent experiments by which Chadwick proved the existence of the neutron earned him the 1935 Nobel Prize in physics.

And today…..the wave-mechanical atom

Orbitals are strange looking things…

And leptons, hadrons, quarks etc………. This story is not yet finished (or is it??)!