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442 BCE
Democritus
Put forth in 442 BC. *.All matter consists of invisible particles called atoms.
*. Atoms are indestructible.
*. Atoms are solid but invisible.
*. Atoms are homogenous.
*. Atoms differ in size, shape, mass, position, and arrangement. Summary-
He theorized that atoms cannot be destroyed, are different in size, temperature and shape. He also believed that atoms are infinite. -
John Dalton
Put forth in 1803 *.All elements are made up of atoms. It is impossible to divide or destroy an atom.
*.All atoms of the same element are alike.
*.All atoms of different elements are different.
*.Atoms of different elements combine to form elements, these atoms have to be whole number ratios.
*.chemical reactions occur when atoms are separated, joined or rearranged. Summary-
Matter consists of undividible atoms. His atomic theory said that elements consisted of tiny particles called atoms. -
JJ Thompson
Put forth in 1904. *.Discovered electrons and that an atom can be divided.
*.Concluded that atoms are made of positive cores and negatively charged particles within it. Summary-
His model and discoveries helped people better understand an atom and its generic makeup. He created the "plum pudding model" -
Gold Foil Experiment
Ernest Rutherford performed a famous experiment called the "gold foil experiment". He took a thin sheet of gold foil, when used special equipment to shoot alpha particles at the gold foil. Most particles passed straight through the foil like the foil was not there. Some particles went straight back or were deflected as if they had hit something. -
Robert Millikan
Put forth in 1909 *.conducted the "the oil drop experiment" to determine the charge of an electron.
*.By repeating the oil drop experiment, he confirmed that the charges were multiples of some fundamental value. Summary-
His oil drop experiment helped increase the charge of an electron. This greatly contributed to our understanding of the structure of an atom and the atomic theory. -
Ernest Rutherford
Put forth in 1911 *.Proposed that atoms have a small dense center filled with positive charges.
*.He named this center the nucleus. He then said that negatively charged electrons surrounded the dense, positively charged center. Summary-
Atoms are made of a small positive nucleus because positive nucleus repels positive particles, atoms are mostly empty space. -
Neils Bohr
Put forth in 1913 *.Proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in defined quantities
*.Electrons move around the nucleus in prescribed orbits.
*.When jumping from one orbit to another with lower energy, a light quantum is emitted. Summary-
his concept could explain why atoms emit light in fixed wavelengths. -
Irwin Schrödinger
Put forth in 1926 *.Developed the "Electric Cloud Model".
*.A cloud of electrons at different levels in orbit.
*.Formed a wave equation that accurately calculated the energy levels of electrons in atoms. Summary-
Discovered that electrons don't move in orbits. He theorizes electrons move in waves, and that they have no exact location. -
Werner Heisenberg
Put forth in 1927 *.Proposed the "uncertainty relation", setting limits for how accurately the position and speed of a particle can be determined.
*.Formulated a type of quantum mechanics based on matrices Summary-
Stated that the speed and position of an atom can not be judged at the same time. -
James Chadwick
Put forth in 1932 *. bombarded beryllium atoms with alpha particles, unknown radiation was produced.
*.He interpreted this radiation as being composed of particles with a neutral electrical charge and the mass of a proton.
*.This particle became known as the neutron. With the discovery of the neutron, an adequate model of the atom became available to chemists. Summary-
James Chadwick discovered the neutron and with that, an adequate model of an atom was present.
