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Democritus (BC)
The exact date of his development of atomic theory is unknown. Democritus of Adbera was an ancient Greek philosopher, who conceived the idea that matter was made up of minuscule, irreducible particles, which he called atoms, a name derived from the Greek word atomos, meaning indivisible.) He said this about it: "Nothing exists except atoms and empty space. Everything else is opinion" Democritus' conception of the atom was, we now know, incorrect - the atom is not indivisible. -
Isaac Newton
Proposed a mechanical universe with small solid masses in motion. -
John Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass -
Michael Faraday
Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism. -
G.J. Stoney
Proposed that electricity was made of discrete negative particles he called electrons -
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. -
Joseph Thomas
Thomson suggested that the model of an atom as a sphere of positively charged matter with negatively charged electrons surrounding them. He stated that electrons were positioned by electrostatic forces. -
Frederick Soddy
Observed spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay -
Marie and Pierre Curie
Showed that the atom was divisible. This was significant as up to that time, from Dalton's theory, atoms were thought to be indivisible. So this was an important step toward understanding the structure of the atom. -
Hantaro Nagaoka
Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle. -
Hans Geiger
Developed an electrical device to "click" when hit with alpha particles -
R.A. Millikan
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
Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure. -
Niels Bohr
Niels Bohr applies quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits defined by their angular momentum. This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit. -
H.G.J. 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. -
Francis William Aston
Discovered the existence of isotopes through the use of a mass spectrograph. -
Erwin Schrodinger Atomic Theory
Erwin Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position in atomic theory. This atomic model is known as the quantum mechanical model of the atom. This atomic theory predicts the odds of the location of the electron. -
James Chadwick
James Chadwick was the first man to discover the neutron-the neutrally charged particle of the nucleus (which also contains protons, positively charged particles). Electrons whizz around the nucleus (which is the heaviest part by far of the atom) on "orbits". -
Enrico Fermi
Conducted the first controlled chain reaction releasing energy from the atoms nucleus. -
Aristotle (BC)
Aristotle, as a matter of fact, did not believe in atoms. He came up with his own theory about the nature of things which, however, did not prove right. Some other Greek philosophers, however, did believe in atoms.
