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400
Democritus proposed that there are atoms and void
Democritis is a Greek scientist whoose discovery, in 400 BCE, lead him to theorise that everything is made up of void and atoms. Depending on the arrangement, the density varies with less void being denser. His discovery has helped the development of the modern atomic theory. He quoted, "by convention bitter, by convention sweet, but in reality atoms and void." -
Isaac Newtown proposed a mechanical universe with small solid masses in motion.
Isaac Newton was an English scientist who propsed that there are short range electrical forces that hold atoms together and these forces are now understood by Quantum mechanics. -
John Dalton proposed an "atomic theory" with spherical solid atoms
John Dalton was an English scientist who theorised that chemical element atoms have a unique atomic weight and can be placed in order on what is known today as the periodic table of elements. -
G.J. Stoney proposed that electricity was made up of “electrons ".
G.J. Stoney was an Irish scientist who proposed that electricity was made of discrete negative particles he called “electrons " and that these "electrons" were found in atoms. -
J.J. Thomson concluded that there were small, electrically, negatively charged particles that are 1000 times less that a hydrogen atom.
J.J. Thomson, an English scientist, performed a series of experiments on cathode rays. He shone a beam of the cathode ray light into a cathode ray tube and found it was attracted to the positive charge and repelled by the negative charge. He concluded that the rays consisted of small, electrically, negatively charged particles that are 1000 times less that a hydrogen atom. -
Max Planck used the idea of what he called quanta to explain the radiation from a radiating atom.
Max Planck, a German scientist, helped to create the Quantum theory which deals with the particles that atoms are made of (subatomic particles). It explains that energy is always absorbed and emitted from atoms in discrete units called quanta. -
Hantaro Nagaoka created a model of the atom and explained how atoms kept elctrons in orbit
Nagaoka, a Japanese scientist, assumed a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle. Also explained how forces in atoms kept electrons in orbit around the nucleus by attracting an drepelling forces. -
Robert Milikan determined charge and mass of an electron
Robert Milikan, an American scintist, performed the oil drop experiment and determined charge and mass of an electron. HIs experiment showed that electrons are negatively charged and that they each have a mass of 1.602 x 10-19 Coulombs. -
Ernest Rutherford demonstrated that the atom has an internal structure of a small, dense nucleus about which negatively charged electrons circle in orbits.
Ernest Rutherford, an English scinetist, demostrated through an experiment using alpha particles and very thin gold foil, that the atom is not as solid as previously believed but that it has an internal structure of a small, dense nucleus about which negatively charged electrons circle in orbits. -
H.G.J. Moseley discovered mathematical relationship between wavelengths of X-ray and the atomic number. He also determined the charges on the nuclei of most atoms.
H.G.J. Moseley, an english scientist, performed an experiment using diffraction through crystal, he discovered a mathematical relationship between wavelengths of X-ray and the atomic number. He also determined the charges on the nuclei of most atoms. -
Niels Bohr developed an explanation of atomic structure that agrees with the periodic table of elements.
Niels Bohr, a Danish scientist, developed an explanation of atomic structure that agrees with the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. He suggested that the outer shells hold more than the inner ones and that they control the chemical properties of the atom. -
Paul Dirac proposed anti-particles and put forward a theory of electrons and a theory of holes.
Paul Dirac, an English scientist, proposed anti-particles and put forward a theory of electrons and a theory of holes. Also that there were other particles in an atom that had the same mass and charge as the electron. These were later discovered by another scientist C.D. Anderson, and were named positrons. -
James Chadwick discovered the neutron.
James Chadwick, an English scientist, used alpah particles in an experiment and discovered a neutral atomic particle with a mass close to a proton. Thus the neutron was discovered. -
Enrico Fermi discovered new radioactive elements produced by neutron irradiation
Enrico Fermi, an Italian scientist, discovered that slow neutrons in the nucleus are what create nuclear reactions. Also by exposing the neutrons to X-rays and radiation, radioactive elements are produced.
