The Atomic Theory of Matter
The understanding of the nature of matter as atoms was first set down by John Dalton (1766-1844). Dalton lived in Manchester, England, and from 1793 earned his living as a private tutor there. In the course of his studies and teaching, as well as in discussions with other members of the Literary and Philosophical Society of Manchester, he developed the ideas which led to his formulation of the atomic theory of matter in 1805.
Quantitative chemical measurements, especially those of mass, had given rise to several empirical laws which preceded the theoretical basis given to them by John Dalton It was to explain these empirical laws that he developed the atomic theory of matter.
Probably the best-known and most widely accepted of the empirical laws known to Dalton was the law of conservation of (total) mass, which had been used by A. L. Lavoisier in 1798. The law of conservation of mass can be stated as follows: The total mass of the reactants in any chemical reaction is exactly equal to the total mass of the products.
The second empirical law known to Dalton was the law of constant composition of compounds. This law is a statement of the observed fact that any pure substance has a fixed composition in terms of the chemical elements.
A theory in many cases will not only explain known facts but will also predict new ones. The empirical law of multiple proportions was apparently developed by Dalton himself around 1804. The law of multiple proportions can be stated as follows: When any two elements are observed to form more than one compound between them, the mass ratios in one compound will be related to the mass ratios in the other in the proportions of small whole numbers.
We now know that Dalton's original idea that atoms were small, indivisible hard particles of various types is an oversimplified picture. Experiments conducted around the beginning of the twentieth century showed that atoms themselves consist of particles.
The Structure of the Atom
In 1897 the British physicist J. J. Thompson carried out a series of experiments that showed that atoms were not indivisible particles. Thompson used an apparatus similar to that shown here, which is a cathode ray tube, which is the forerunner to the television picture tube.
From his experiments, Thompson concluded that a cathode ray consists of a beam of negatively charged particles (electrons) and that electrons are constituents of all matter. He could also calculate the ratio of the electron's mass, me, to its electric charge, e. He could not obtain either the mass or the charge separately.
In 1909 U. S. physicist Robert Millikan performed a series of experiments in which he obtained the charge on the electron by observing how a charged droplet of oil falls in the presence and absence of an electric field. The charge on the electron has been found to be 1.602 x 10-19 coulombs.
Even before neutrons were discovered, scientists were wondering how electrons and protons were put together in an atom. The prevailing theory was that the protons and electrons were evenly distributed throughout the volume of an atom. In 1911, Ernest Rutherford (1871-1937) and his co-workers at the University of Manchester, England, decided to test this theory of atomic structure. For their test they chose alpha particles. Alpha particles are helium atoms that have lost two electrons and have a double positive charge from the remaining two protons. In their experiment, they directed a narrow beam of alpha particles at a very thin sheet of gold foil. According to the existing theory, they expected the alpha particles to pass straight through the gold. To everyone's surprise, a small fraction of the alpha particles deflected, or bounced off, the gold foil at very large angles, A few alpha particles even bounced straight back toward the source. Based on the experimental results, Rutherford suggested a new theory of the atom. He proposed that almost all the mass and all the positive charge are concentrated in a small region at the center of the atom. He called this region the nucleus. The nucleus is the central core of an atom, composed of protons and neutrons. Because protons and neutrons have a much greater mass than electrons, almost all of the mass of an atom is concentrated in a tiny nucleus. The nucleus is so dense that if it were a cube one centimeter on each side its mass would be 250 million tons!
