Chapter+2

ATOMIC THEORIES

Democritus defined the atom as an indivisible particle.

Dalton's theory: Each element is made up of tiny particles, atoms. The atoms of any element are identical-- the atoms of different elements are different in a fundamental way. Compounds are formed when atoms of different kinds combine with each other. Any given compound has the same relative numbers and types of atoms. Chemical reactions involve reorganization of the atoms.

Thomson studied electrical discharges in partially evacuated tubes called cathode ray tubes. He found that when high voltage was applied to the tube, a ray, emanating from the negative electrode was produced. He theorized that this was a stream of negative particles.

Thomson did his experiments to try to gain an understanding of the structure of the atom. Thomson's model was the plum pudding model, which depicted a spherical cloud of positive charge with negative electrons embedded randomly in it.

Rutherford later conducted an experiment in which he directed alpha particles at gold foil, saying that if the plum pudding model were true, the alpha particles would dash through the gold foil like cannonballs, with at most minor deflections to their paths. Most of the alpha particles, however, passed straight through, many particles were deflected at large angles. Rutherford concluded that the deflections could only be caused by a large center of concentrated positive charge that contained almost all of the atom's mass-- the rest of the atom, he concluded, must be empty space.