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Democritus's Atomic Philosophy- (460 b.c.-370 b.c.)
Among the first to suggest the existence of atoms. He reasoned that atoms were indivisible and indestructible. Although his ideas agreed with later scientific theory, they did not explain chemical behavior. They also lacked experimental support because his approach was not based on the scientific method. -
Dalton's Atomic Theory (1776-1844)
By using experimental methods, Dalton transformed Democritus's ideas on atoms into a scientific theory. Studied the ratios in which elements combine in chemical reactions. Based on the results of his experiments, Dalton formulated hypotheses and theories to explain his observations; known as Dalton's atomic theory. According to the theory, an element is composed of only one kind on atom, and a compound is composed of particles that are chemical combinations of different kinds of atoms. -
Protons- Eugen Goldstein (1850-1930)
Eugen Goldstein observed a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He called these rays canal rays and concluded that they were composed of positive particles. Each proton has a mass about 1840 times that of an electron. -
Electrons- Thompson (1856-1940)
English physicist J.J. Thompson discovered the electron. He performed experiments that involved passing electric current through gases at low pressure. The result was a cathode ray. He hypothesized that a cathode ray is a stream of negatively charged particles moving at high speed. He named these particles corpuscles; later were named electrons. He concluded that electrons are a component of the atoms of all elements. -
Electrons Charge- U.S. Physicist Robert A. Millikan (1868-1953)
Millikan carried out experiments to find the quantity of an electron's charge. From his data, he found that the charge on each oil droplet was a multiple of 1.60 x 10^-19 coulomb, meaning this must be the charge of an electrons. Using the charge value and Thompson's charge-to-mass ratio of an electron, Millikan calculated an electron's mass. His values for electron charge and mass are similar to those accepted today. -
The Rutherford Atomic Model- Ernest Rutherford (1871-1937)
Rutherford and his co-workers wanted to test the existing plum-pudding model of atomic structure. They devised the gold-foil experiment. Based on the experimental results, he suggested a new theory of atom. Proposed that the atom is mostly empty space-the nucleus. The atomic model is known as the nuclear atom. In the nuclear atom, the protons and neutrons are located in the positively charged nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom. -
Neutrons- James Chadwick (1891-1974)
The English physicist James Chadwick confirmed the existence of yet another subatomic particle: the neutron. Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.