Atomic Theory: Substances and elements are made up of small units called atoms. Matter could be subdivided into these fundamental units and no further.
Democritus. Ancient Greek philosopher who advocated the atomic theory of matter.
Lavoisier. Preformed the classic experiment with mercuric oxide. By heating mercuric oxide (HgO) and allowing it to decompose into its component elements he discovered that the component elements mercury and the gas (oxygen) given off always combine in the same ratios by weight to form the compound. In this case the ratio by weight of Hg to O was always 12.5 to 1.
Dalton. Formalized the discovery of Lavoisier into the "Law of Definite Proportions:" When atoms combine to form a particular compound, they always combine in the same ratios by weight.
Avogadro's law. Gases (under ideal conditions) combine to form compounds in the same ratios by volume; equal volumes of gases under the same conditions of temperature and pressure contain the same number of particles.
Mendeleev. Discovered a periodicity in the properties of elements.
The Periodic table of Elements. The elements arranged in families, ordered in order of increasing atomic number (the number of protons in the nucleus).
Brownian Motion. The jerking movement of microscopic particles only visible under a high powered microscope. It was discovered by the biologist Robert Brown in 1827. It is caused by the molecular movement of the molecules of the fluid in which the particle is suspended. Einstein analyzed and explained the phenomenon in 1905.
J. J. Thomson. Studied the newly invented cathode ray tube (a TV screen is a cathode ray tube). He showed that atoms could be further subdivided into negative (which he named electrons) and positive components. He postulated a "Plum Pudding" model for atoms. He calculated the charge to mass ratio e/m for the electron by careful observations of the curvature of an electron beam in cathode ray tubes in a magnetic field.
Millikan. He calculated the charge on the electron with his famous oil drop experiment. He measured the static electrical charge on microscopic oil droplets by balancing droplets between charged plates.
Ernest Rutherford. In a classic experiment he bombarded a thin sheet of gold foil with alpha particles (He nuclei: 2 protons + 2 neutrons). He discovered that the particles bounced off of something dense in the foil. From this experiment he postulated that atoms are formed of a small dense positively charged nucleus "orbited" by negatively charged electrons.
Marie and Pierre Curie. Did considerable work in the investigations of radioactivity. Marie is the first woman to win a Nobel Prize. They discovered two new elements: Polonium and Radium - the latter named for the radioactive glow visible in the dark.
Niels Bohr. Developed the modern quantum mechanical model of the atom.
Electron orbitals. The regions around the nucleus of the atoms where the electron "probability wave" resides. These replace Rutherford's model of electron orbits which, according to Maxwell's laws of electro-magnetism, would be unstable.
Energy levels and spectra. When electrons jump from one orbital to another they emit or absorb radiation in fixed units called quanta.
Max Plank. Discovered that electro-magnetic radiation is emitted in fixed units or packets which he called quanta.
Heisenberg. Formulated the uncertainty principle.
Uncertainty Principle. The principle states that there are certain "complementary" quantities of particles such as position and momentum. These quantities are correlated so that the product of the errors of measurement must be greater than Planks constant.