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442 BCE
Democritus
Democritus put forward the atomic model in 442 BC. He stated that atoms are indestructible and unchangeable; Also that they are homogeneous. His atomic model stated all atoms differ in size, shape, mass, position and arrangement, with an existing void between them. He reasoned that if you continued cut a stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. Democritus theorized that there was a stopping point called the atom. -
John Dalton: September 6, 1766-July 27, 1844
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 have atoms of differing size and mass. Also stated that all compounds were composed of combinations of these atoms in definite ratios. He developed the law of multiple proportions by studying and expanding. He also believed atomic theory could explain why water absorbed different gases in different proportions. -
John Dalton: The first evidence based atomic theory
Democritus was the first person to have an atomic theory, but he had no evidence. Then John Dalton was able to find evidence to support the atomic theory. 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. -
Cathode Ray tube experiment
Thomson used a magnet to bend the cathode rays away from the meters. When he did this, he discovered that the meters stopped measuring electric charge. Next, J.J. Thomson used an electric field. He put a negatively charged metal plate on one side of the cathode rays let out past the anode, and a positively charged metal plate on the other side. Instead of a meter, he now had a fluorescent coating where the cathode ray hit it. This showed that the charge carried by the cathode rays was negative. -
J.J. Thomson: December 18, 1856-August 30, 1940
J.J. is credited with the discovery and identification of the electron by his Cathode Ray tube experiment 1897. In 1904, he did the plum pudding model of the atom as a sphere of positive matter with electrons positioned based on electrostatic forces. He took the deflection of the rays by the magnets and charged plates as evidence of 'bodies much smaller than atoms'. He then declared these tiny negatively charged particles were called “electrons”. -
Ernest Rutherford: August 30, 1871-October 19, 1937
Rutherford established the nuclear theory of the atom with his gold-foil experiment. When he shot a beam of alpha particles at a sheet of gold foil, a few of the particles were deflected. He realized that a tiny, dense nucleus was causing the deflections. This caused Rutherford to conclude that the mass of an atom was concentrated at its center. -
Gold foil experiment
The Rutherford Gold Foil experiment shot minute particles at a thin sheet of gold. It was found that a small percentage of the particles were deflected, while a majority passed through the sheet. This caused Rutherford to conclude that the mass of an atom was concentrated at its center. -
Oil drop experiment
He sprayed tiny drops of oil into a chamber. He measured how fast the drops fell under the force of gravity;He could then calculate the mass of the individual drops. Then he sprayed oil drops and applied an electrical charge to them. The oil drops picked up static charge and were suspended between two charged plates. He was able to observe the motion of the oil drops and found that the drops lined up in a specific way based on the number of electric charges they had acquired. -
The size of the charge on an electron
From 1909 to 1910, Robert Millikan used an oil drop experiment to find the size of the charge on an electron. He carefully studied individual droplets, which enabled him to find that the charge on a drop always varied, but only by a small amount. In his oil drop experiment, he used a atomizer from a perfume bottle to spray water or oil into a sample chamber. Some of the droplets fell down a pinhole between two plates of an electric field, where he could observe them with a microscope. -
Robert Millikan : March 22, 1868-December 19, 1953
His oil drop experiment helped to quantify the charge of an electron. He worked with Harvey Fletcher, to attempt to measure the charge of an electron. By carefully studying individual droplets, Millikan was able to show that the charge on a drop was always an integral multiple of a small value. In 1910, Millikan determined the charge of an electron using the oil-drop apparatus. -
Ernest Rutherford: Discovery of the nucleus
Ernest Rutherford discovered the nucleus by doing a gold foil experiment. He observed that every atom contains a nucleus where all of its positive charge and most of its mass are concentrated. -
Neils Bohr: October 7, 1885-November 18, 1962
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. It could explain why atoms emitted light in fixed wavelengths. His shell model of the atom explained how electrons can have stable orbits around the nucleus. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump. It is now known as Bohr model of the atom. -
Erwin Schrodinger: August 12, 1887-January 4, 1961
Schrodinger lived in Vienna and Dublin throughout his life. In 1926 Schrodinger used mathematical equations to describe the likelihood of finding an electron in a certain position. He concluded that it was a wave of motion. This atomic model is known as the quantum mechanical model of the atom. His discovery was a result of his dissatisfaction with the quantum condition in Bohr's orbit theory and his belief that atomic spectra should really be determined by some kind of problem. -
Erwin Schrodinger continued
Erwin Schrodinger and Paul Adrian won the Nobel Prize in physics because they discovered new forms of the Atomic theory. Then, in 1940, he was brought over to Ireland where he helped establish the Institute of Advanced Studies. After the school was finished, he became the director for theoretical physics. In 1944, Erwin wrote the book "What is Life?" -
James Chadwick: Discovery of a neutron
James Chadwick put a piece of beryllium in a vacuum chamber with some polonium. The polonium emitted alpha rays, which struck the beryllium. When struck, the beryllium emitted the mysterious neutral rays. In the path of the rays, Chadwick put a target. When the rays hit the target, they knocked atoms out of it. The now electrically charged atoms then flew into a detector. He counted the atoms and estimate their speed. The only good explanation for his result was a neutral particle, the neutron. -
James Chadwick: October 20, 1891- July 24, 1974
In 1932, James Chadwick bombarded beryllium atoms with alpha particles.Chadwick interpreted the radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton. Which he then discovered was a neutron. He was a British physicist who was awarded the 1935 Nobel Prize in Physics for his discovery of the neutron in 1932. In 1941, he wrote the final draft of the MAUD Report, which inspired the U.S. government to begin serious atomic bomb research efforts.
