atomic theory: a theory that explains behavior of chemical elements based on the atoms that they are made of, and the composition of those atoms.
Suggestion: you need to memorize these scientists and their contributions to atomic theory. Making a set of flash cards may be helpful.
Democritus: ancient Greek philosopher. Credited with the concept that everything is made of something, and there is a “smallest piece” of matter, which is called an atom.
John Dalton: British teacher & scientist from the early 1800s. Credited with the first theory of what atoms are and how they behave:
Dalton’s Atomic Theory:
· everything is made of atoms
· atoms of the same element are identical
· atoms of different elements are different
· atoms are not created or destroyed in chemical reactions. Chemical reactions are simple rearrangements of the atoms into different compounds.
· every sample (molecule) of a compound contains the same atoms in the same proportions (“Law of Constant Composition”)
J.J. Thompson: discovered the electron. His experiment was to apply an electric current to a gas. This produced rays of electricity from the cathode (called cathode rays). These negatively-charged cathode rays were thought to be “particles of electricity,” or “electrons.”
Robert Millikan: measured the charge on an electron by measuring electrical charge based on the rate that oil drops fall through an electric field. The common factor in all of the measurements must be the basic particle of electric charge—the electron.
Lord Kelvin: known for the “plum pudding” model (theory) of the atom. He described an atom as being like a bowl of plum pudding with raisins. The “pudding” was positively charged, and it contained “raisins,” which were the negatively-charged electrons.
Ernest Rutherford: discovered the nucleus in his “gold foil experiment.” He passed positively-charged particles through a thin sheet of gold foil. Most of the particles passed through; some were deflected slightly. However, a few were deflected sharply, as if they had interacted with a dense object with a strong positive charge. Conclusions:
· Most of the atom is empty space.
· The atom has a dense, positively-charged core in the center. He called it the “nucleus,” because it reminded him of the nucleus of a cell.
Rutherford described the atom as being like a miniature solar system. This “planetary” model of the atom was popular for several years, and is frequently taught today in elementary and middle school science classes.