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400 BCE
Democritus (460 BC-370 BC) Greek Philosopher
Born in Abdera, Greece around 460 BC., Democritus proposed the first evidence based atomic theory by saying that all things are made of “atoms” which are physically indestructible, there is empty space between the atoms, the atoms are always in motion, and that there are infinite numbers and kinds of atoms. He created this hypothesis, by breaking a seashell in half over and over until he found the smallest particle. He was also instructed in Pythagoreanism and a disciple of Leucippus. -
John Dalton (1766-1844) British Chemist
John Dalton made his great discovery that all matter was made from atoms by experimenting with how water absorbs some gases more than others. He hypothesized that the differences were due to the different mass and complexity of the gas particles. He was correct, with the CO2 being heavier than Nitrogen. He then proposed that each element is made up of unique atoms that are indestructible. He also studied color-blindness and named his studies “Daltonism”. He defined an atom to be ball-like. -
J.J. Thomson (1856-1940) British Physicist
Thompson studied and taught at Cambridge University. He is most known for discovering the electron by his Cathode Ray tube experiment. Electrons flowed off the cathode and are attracted toward the anode. This created a beam of cathode rays that were negatively charged electrons. He also proposed the plum pudding model of the atom, which is a sphere of positive charge, and negatively charged particles are inside it to balance the charges out. He worked with and taught Ernest Rutherford. -
Robert Millikan (1868-1953) American Physicist
Millikan worked at the University of Chicago and lived there.He is credited with the discovery of the size of the charge on an electron through his oil drop experiment in 1909. He put a charge on a tiny drop of oil, and measured how strong an electric field had to be in order to stop the oil drop from falling. He found out that the electrons were carrying this charge. He discovered the quantity of the charge and that it is constant for all electrons. In 1923, received a Nobel Prize for his work. -
Ernest Rutherford (1871-1937) British Physicist
Rutherford studied in Cambridge,Eng. He was the 1st to prove that atoms have a charged nucleus surrounded by electrons through his gold foil experiment. He shot particles through a sheet of gold and found that a small percentage of the particles were deflected, but many passed through the sheet. He saw the mass of an atom was dense at the center, proving the existence of nucleus. Based off the theory, he invented the well-known Rutherford or planetary model in which electrons orbit the nucleus. -
Niels Bohr (1885-1962) Danish Physicist
Born in Copenhagen on October 7th 1885. In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities and orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths. -
Erwin Schrodinger (1887-1961) Austrian Physicist
He was a Nobel Prize-winning physicist who developed a number of fundamental results in quantum theory to provide a way to formulate a wave equation that calculated the energy levels of electrons and how it changes overtime. His discovery started with his displeasure in the quantum condition of Bohr’s orbit theory, so he came up with a new idea for the model. This model was called the Quantum Mechanical Model of the Atom, and can be portrayed as a nucleus surrounded by an electron cloud. -
James Chadwick (1891-1974) British Physicist
Lived in the UK & awarded with the Nobel Prize for his discovery of the neutron. He discovered the neutron by using kinematics and was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton. He also wrote the final draft of the MAUD which inspired the gov to begin serious atomic bomb research. His model is known as the quantum mechanical model.
