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Democritus
A Greek phlisopher born in 640 B.C.E. He created the theory there was a smallest part of everything,which he called an atom. He believed that they were indivisible, moved around freely in space, and couldn't be created or destroyed. Denocritus created a model of his 'invisible sphere' which was just a wooden sphere. The atomists' work was generally accepted until Aristotle. -
Aristotle
Born around 469 B.C.E., Aristotle disagreed with the atomists' view of matter. He couldn't see how atoms could continously move through a void. Instead, he claimed everything was made of the four elements - earth, air, water and fire. These four elements had two properties each: wetness, dryness, hotness and coldness. Therefore, air was hot and wet, earth was cold and dry, water was wet and cold, and fire was hot and dry. This theory held until the scientific revolution in the late 1700s. -
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Atoms through the ages
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Antione Lavoisier
Through his experiments, he discovered the law of conservation of mass, which states that matter cannot be created or destroyed. This simply built upon the philosophy of Democritus. With his studies on combustion, he realized that some things could not be taken apart, which he called elements. He put these elements into a chart, making the precursor to the periodic table. -
John Dalton
With his four postulates of atomic theory, Dalton built on Lavoisier's work by connecting the idea of atoms to elements. He also was one of the first to address compounds, which he said were made up of fixed whole-number ratios of elements. -
Henri Bacquerel
A co-recipient of the 1903 Nobel Prize (with the Cuires), Bacquerel's experiments involved using uranium salts with photographic plates. He noticed that the plates became fogged, which led him to the discovery that uranium radiation differed from X-rays in that it could be affected electric and magnetic fields. He also discovered the process of natural radioactivity. -
Max Planck
One of the greatest recognized physists, Planck formed the basis of quantum theory. The constant he discovered, known as Planck's constant, can be used to find the energy of a photon. He maintained that light can act as a particle as well as a wave. He claimed that energy did not move in a constant, but in little 'packets', which he called quata. He also discovered the process of heat radiation. -
Marie & Pierre Curie
Building on the work of Becquerel, the Curies discovered that the strength of radioactivity of a substance didn't depend on how the chemicals were linked but only on how much of the radioactive elements were in the compound. They also isolated the elements radium and polonium. -
J.J. Thomson
Using an experiment involving cathode-ray tubes, Thomson discovered magnets had an effect on the rays, Because only matter can be affected by magnets, Thomson theorized that there must be particles smaller that the atom that have a large charge-to-mass ratio. These were called electrons. With this he created his plum pudding model, which showed the atom as a mixture of electrons and protons scattered in the atom. -
Ernest Rutherford
A student of Thomson, Rutherford set out to prove his mentor right in his famous gold foil experiment, which shot alpha particles at gold foil. He was shocked when a small portion of the particles were reflected back. This led Rutherford to the discovery of the nucleus, which disagress with Thomson's plum pudding model because it shows a concentration of protons and neutrons in the center of the atom. -
Niels Bohr
Expanding on the work of many others, includin Planck, Rutherford and Thompson, Bohr sucessfully created a model of the atom that represented the different energy levels, or orbitals, and his new theory of angular momentum. This differed from Rutherford's model becuase it displays energy levels that prevent electrons from spiraling into the nucleus. It is perhaps the most common model in use today, although it is incorrect. -
Henry Moseley
He discovered that the emission of X-rays varied linearly with each consecutive element on the periodic table. This caused a rearangement of the elements according to protons in the nucleus as opposed to atomic mass, leaving more blanks for elements soon to be discovered. -
Robert Millikan
Millikan created his law of motion (building on Newton's work) of falling particles in Earth's atmosphere and extended the limit of the ultraviolet spectrum. His greates contribution to atoms, however, was discovering electrons' charge and mass, thereby allowing others to find the number of protons based on the number of electrons in a stable elemental atom. -
Werner Heisenberg
Building on the work of Bohr and Schrodinger, among others, Heisenberg is best known for his principle (the Heisenberg uncertainty principle), which restricts the accuracy of some properties of atoms and particles, such as velocity and position. Therefore, these electrons created a cloud, with no discernable position. This contrasts the Bohr model, which explicitly shows the orbitals of elelctrons. -
Erwin Schrodinger
As critics found more and more problems with Bohr's atomic model, Schrodinger set out to create an atomic represenation that agreed with experimental evidence. This led him to discover Schrodinger's wave equation, which explained the contrasts with Bohr's model. -
James Chadwick
Building on Rutherford's discovery of the nucleus, Chadwick discovered a third particle in the atom: the neutron. This neutrally charged particle holds the atom together by nuetralizing the charges of the protons and electrons. He made possible the discovery of nuclear fission with neutrons and the invention of the atomic bomb.
