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460
Democritus
460B.C.
The theory of Democritus said that everything is composed of "atoms". Which are indivisible; that between atoms lays empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. -
John Dalton
Dalton formed the atomic theory.
1.Elements are made of extremely small particles called atoms.
2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
3.Atoms cannot be subdivided, created, or destroyed.
4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
5.In chemical reactions, atoms are combined, separated, or rearranged -
Eugen Goldstein
Goldstein discovered rays in a cathode ray tube traveling the opposite direction of the cathode rays. These particles are called protons. Protons are positively charged subatomic particles. Goldstein also figured that protons have a mass about 1840 times that of an electron. -
J.J. Thomson
An English physicist who discovered the electron. Thomson said that electrons are negatively charged subatomic particles. Thomson passed an electric current through gases in a sealed glass tube. When he did this a cathode ray traveled from the cathode to the anode because the negatively charged particles go to the positively charged anode. Thomson discovered that a cathode ray is deflected by an electrically charged plate. This proved that the ray had a negative charge. -
Max Planck
He was especially interested in the nature of radiation from hot materials. In 1901 he devised a theory that perfectly described the experimental evidence, but part of it was a radical new idea: energy did not flow in a steady continuum, but was delivered in discrete packets Planck later called quanta. That explained why, for example, a hot iron poker glows distinctly red and white. Planck, a conservative man, was not trying to revolutionize physics at all, just to explain the particular event. -
Robert A. Millikan
Millikan put a charge on a tiny drop of oil, and measured how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. -
Ernest Rutherford
Theorized that atoms have their positive charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model of the atom, through his discovery and interpretation of Rutherford scattering in his gold foil experiment.
Rutherford's conclusions:
-atoms are mostly empty space
-the positive charge and most of the mass of the atom are concentrated in a small region
-the nucleus is the tiny core of an atom and is composed of neutrons and protons. -
Niels Bohr
Bohr published his model of atomic structure in 1913. Here he introduced the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory. -
Wolfgang Pauli
Pauli predicted that an electron should spin like a top while it orbits around the nucleus. The electron can spin in either of two directions. This spin consisted of a fourth quantum number: electron spins. Pauli gave a rule that behavior of electrons within the atom agreed with experiment. If an electron has a certain set of quantum numbers, then no other electron in that atom can have the same set of quantum numbers. Physicists call this "Pauli's exclusion principle." -
James Chadwick
Chadwick proved the existence of neutrons - particles without any electrical charge. A neutron is a subatomic particle with no charge, but a mass nearly equal to a proton. This was proved by later more accurate measurement that the mass of neutron was very close to that of proton-only greater 1/1000 than the mass of proton. James Chadwick then concluded his discovery in his paper entitled "The Existence of the Neutron" and published in the Royal Academy Journal.
