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400 BCE
Democritus
According to Democritus atoms were miniscule quantities of matter. He adopted this from his mentor and changed it a little. “The universe is composed of two elements: the atoms and the void in which they exist and move.” -
Isaac Newton
Proposed a mechanical universe with small solid masses in motion. He thought there were little tiny pieces of mass that were 'swimming' everywhere. -
John Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. Dalton's atomic theory proposed that all matter was composed of atoms, indivisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of differing size and mass. -
Dmitri Mendeleev
Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law. His table arranged the known elements according to their chemical properties and in order of their relative atomic mass. -
J.J. Thomson
Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram and studied "canal rays" and found they were associated with the proton H + . He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. -
Marie Curie
Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium. Marie and Pierre Curie were awarded the 1903 Nobel Prize in Physics. -
Hans Geiger
Developed an electrical device to "click" when hit with alpha particles. This was called the geiger counter and he was also known for the Geiger–Marsden experiment which discovered the atomic nucleus. -
R.A. Millikan
His oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory. The oil drop experiment determined the charge (e=1.602 x 10 -19 coulomb) and the mass (m = 9.11 x 10 -28 gram) of an electron. -
Ernest Rutherford
Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. -
H.G. Moseley
Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass. -
Niels Bohr
Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. He proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. -
Heisenberg
Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle. -
James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. Neutrons are located in the center of an atom, in the nucleus along with the protons. They have neither a positive nor negative charge, but contribute the the atomic weight with the same effect as a proton. -
Lisa Meitner
Conducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.
