PRESENTATION OUTLINE
DEMOCRITUS
Democritus was a Greek philosopher who lived from 460 B.C.-370 B.C. He was the first to develop an atomic theory - that atoms were indivisible and indestructible. However, his theories were neither explanatory of chemical behavior nor backed by experimental support, because they were not based on the scientific method.
JOHN DALTON
Dalton (1766-1844), an English chemist and schoolteacher, started the modern process of discovery regarding atoms. He transformed Democritus' ideas into proper scientific theories. With many experiments, Dalton formulated a series of hypotheses and theories on the ratios in which elements combine in chemical reactions, which have come to be known as 'Dalton's atomic theory'.
DALTON'S ATOMIC THEORY
1. All elements are composed of tiny indivisible particles called atoms.
2. Atoms of the same element are identical.The atoms of any one element are different from those of any other element.
3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
J. J. THOMSON
n 1897, Thomson (1856-1940), an English physicist, discovered the electron through the use of a cathode-ray tube. He discovered that a cathode ray can be deflected by electrically charged metal plates. A positively charged plate attracted the ray, while a negatively charged plate deflected it. Since he knew that opposite charges attract, Thomson hypothesized that the cathode ray must consist of tiny, negatively charged particles moving at a high speed, which he called electrons. His model of the atom is known as the plum-pudding model, because the electrons were stuck to a lump of positive charge, like raisins stuck in dough.
ROBERT A. MILIKAN
Milikan, a U.S. physicist, calculated the average charge and mass of an electron using his oil-drop experiment. Using the oil's/electron's charge value, 1.60x10^-19, and Thomson's charge-to-mass ratio, Milikan found that an electron has one unit of negative charge, and that it's mass is 1/1840 the mass of no hydrogen atom.
EUGEN GOLDSTEIN
In 1886, Goldstein observed a cathode-ray tube's rays traveling in the opposite direction from another set of rays. Based on this information, he concluded that the other set of rays was composed of positively charged particles, or protons.
JAMES CHADWICK
Chadwick was an English physician, and in 1932, he confirmed the existence of the neutron - a subatomic particle with no charge, but a mass almost equal to the mass of a proton.
ERNEST RUTHERFORD
In 1911, Rutherford, with several of his colleagues at the University of Manchester, challenged the plum-pudding model by passing alpha particles through a sheet of gold foil. Most of the alpha particles passed through the foil without being obstructed, but several of them bounced off the foil in large angles. Because of this, Rutherford concluded that while most of the atom is empty space, the positive charge and the neutrons are concentrated in a tiny lump in the center of the atom. This tiny lump came to be known as the nucleus. His atomic model is known as the nuclear atom.
NIELS BOHR
In 1913, Bohr, a student of Rutherford, changed his mentor's atomic model to incorporate newer discoveries about how an atom’s energy changes when the atom absorbs or emits light. He proposed that an electron is only found in specific circular paths, or orbits, around the atom's nucleus. His model, however, only worked for atoms with only one electron - in other words, hydrogen.
ERWIN SCHRODINGER
In 1926, Schrodinger used calculations and experiments based on electron motion to formulate a mathematical equation describing the behavior of an electron in a hydrogen atom. Modern quantum mechanics is a result of his equations.
