Wesley Barnhill

Democritus-had a theory of the atomic nature of the physical world, developed from that of Leucippus, is known only through the works of critics of the theory such as Aristotle and Theophrastus. It resolved the question of how a world evidently in a state of flux could nevertheless have an underlying nature that was eternal and unchanging. By positing infinitely small things that remained the same but formed different combinations with each other, Leucippus initially, and Democritus in greater detail, managed to answer the question in a way that has been subject to increasingly successful elaboration ever since. One can trace the physical theory of atoms through Epicurus, Lucretius, and Galileo to modern times.

John Dalton- The English teacher, chemist, and physicist , b. Sept. 6, 1766, d. July 27, 1844, is best known for developing the ancient concept of atoms into a scientific theory that has become a foundation of modern chemistry. He considered himself primarily a teacher and earned his living by teaching and lecturing until 1833, when he was awarded an annual civil pension. A self-taught experimenter, he devised simple but effective apparatus for his well-planned tests. Although authors have emphasized the crudeness of his results, many of his data are remarkably accurate.

Sir Joseph John Thomson- b. Dec. 18, 1856, d. Aug. 30, 1940, is universally recognized as the British scientist who discovered and identified the electron. At the age of 27 he succeeded (1884) Lord Rayleigh as professor of physics at Trinity College, Cambridge, and was named director of its Cavendish Laboratory in the same year, continuing in that position until 1919. Thomson demonstrated (1897) that cathode rays were actually units of electrical current made up of negatively charged particles of subatomic size. He believed them to be an integral part of all matter and theorized a model of atomic structure in which a quantity of negatively charged electrons was embedded in a sphere of positive electricity, the two charges neutralizing each other. For these investigations he won (1906) the Nobel Prize for physics; in 1908 he was knighted.

Sir Ernest Rutherford- b. near Nelson, New Zealand, Aug. 30, 1871, d. Oct. 19, 1937, perhaps more than any other scientist, formed modern-day views concerning the nature of matter.

Rutherford made his greatest discovery in 1909. Shortly after his move to Manchester, he found that a few alpha particles, when bombarding thin metal foils, were deflected from their incident beam through more than 90 deg. "It was almost as incredible," Rutherford later responded in a now-classic statement, "as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you." Early in 1911 he finally announced his version of the structure of the atom: a very small, tightly packed, charged nucleus sprinkled with opposite charges in the mostly empty surrounding void. The deflected alpha particles were those that had come into close proximity with the nucleus and had rebounded in various oblique directions. About the time that Rutherford moved (1919) to Cambridge to succeed Thomson as director of the Cavendish Laboratory, he discovered artificial disintegration--the artificial splitting of the atom--a signal discovery that presaged his entry into the field of nuclear physics. Members of his Cavendish team discovered the neutron and the disintegration phenomena produced by artificially accelerated particles.

Niels Henrik David Bohr- b. Oct. 7, 1885, d. Nov. 18, 1962, is known primarily for his pioneering work in the field of atomic theory. Bohr was born in Copenhagen and was educated at the University of Copenhagen at the time when Max Planck had just begun the development of Quantum Mechanics. After completing his dissertation on the electron theory of metals in 1911, Bohr went briefly to Cambridge and then on to Manchester, England. There he worked under Ernest Rutherford, who in 1911 had published the theory that the atom consisted of a central nucleus orbited by electrons. The problem with this model was that, according to classical electrodynamic theory, the electrons should radiate and therefore lose energy and spiral into the nucleus.

The men that helped dig deeper into the research of the atom were vital to science because they introduced a whole new world to science. The more scientists researched the atom the found out how much the atom influences our everyday lives. These influences have helped from the invention of the atomic bomb to precision surgeries.

The time in history had many things to do with how these scientists took their research to the next level. Growing up in the industrial revolution probably had a huge influence on what and how the scientists took their work. Being apart of that time frame pushed for harder work to be done and they had the tools to do what they needed to do.

1) All matter is made of atoms. Atoms are indivisible and indestructible.

2) All atoms of a given element are identical in mass and properties

3) Compounds are formed by a combination of two or more different kinds of atoms.

4) A chemical reaction is a rearrangement of atoms.

Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element that are known as "isotopes", but isotopes of an element have the same chemical properties.

http://www.iun.edu/~cpanhd/C101webnotes/composition/dalton.html
http://library.thinkquest.org/15567/bio/index.html