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470
Democritus
He was born around 460BC. He proposed this theory between 460 – 370BC. His theory has only survived through secondary sources. He was a citizen of Abdera, or Miletus some people argue. He hypothesized that all matter is composed of tiny indestructible units, called atoms. This idea was motivated by the idea of cutting up matter. He developed the theory that the universe consists of empty space and an almost infinite number of invisible particles called atoms. -
Period: 470 to
Atomic theory from the past to present
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Isaac Newton - England
Newton recognised that there are forces between atoms and that they affect the chemical properties of matter. He helped publicise that atoms affect the chemical properties of matter and helped us understand that atoms are what make everything work. -
John Dalton - England
John Dalton presented his theory to the Royal Institution. Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. Using his theory, Dalton rationalised the various laws of chemical combination which were in existence at that time. His theory provided a logical explanation of concepts, and led into new fields of experimentation. -
George Johnston Stoney - Ireland
He theorised that there should be a fundamental unit of electricity associated with atoms. This unit was what he called the electron. Scientists had adopted the word electron to refer to anything with an electric charge. -
Joseph John Thomson - England
What Thomson discovered was that cathode rays were streams of negatively charged particles with a mass about 1,000 times smaller than a hydrogen atom. He claimed that these particles, which he called corpuscles, were the things that atoms were made from. His method based on that theory reveals about the separating of different kinds of atoms and molecules by the use of positive rays. -
Max Planck - Germany
He was a theoretical scientist who was the originator of the quantum theory. He discovered the law of heat radiation, which is named Planck's law of black body radiation. This law became the basis of quantum theory. Planck made the assumption that energy was made of individual units, or quanta. This quantum theory explains the nature and behaviour of matter and energy on the atomic and subatomic level. -
Hantaro Nagaoka - Japan
Eventhough some atom models were proposed in the world of physics in the early 20th century, it was Hantaro Nagaoka, the professor at the University of Tokyo, who was the first to present a Saturnian atomic model close to the presently accepted model. Nagaoka was the first to defend a Saturnian atomic model that looked like Niels Bohr’s atomic model in that the electrons orbit like Saturn’s rings around the nucleus which itself contains a positive electric charge. -
Robert A. Milikan - United States of America
In 1909 Milikan conducted experiments to find the electrical charge of 1 electron. His results were inaccurate and dismissed, then in 1910 he conducted the famous oil-drop experiment. Oil drops are given an negative charge then they fall through a metal plate. Having an accurate idea of the charge of an electron gave greater insight into the make up of the atom in general, greatly advancing the atomic theory and model. -
Ernest Rutherford - New Zealand
Experiments with alpha rays led him to describe the atom as a small, heavy nucleus with electrons in orbit around it. His nuclear model of the atom became the basis for the one that is still accepted today. In 1911, Rutherford deduced from these results that almost all of the mass of an atom, is concentrated in a nucleus a thousand times smaller than the atom itself. The nuclear model of the atom had been born. -
H.G.J. Moseley - England
The physicist H.G.J. Moseley conducted a series of experiments with cathode rays. He found that the heavier the atomic mass of the element, the shorter was the wavelength and the more penetrating were the X-rays. The pattern that emerged when the observed X-rays were organized in order of increasing frequency suggested to Moseley a regular increase in the positive charge on the nuclei. In passing from one element to the next in the periodic table, the atomic number always varied by one unit. -
Neils Bohr - England
With the help of Bohr's principle of correspondence, to establish, in the most important points, the situation of the various tracks of electrons in other atoms. It is on the positions of the outermost electron tracks that the chemical properties of the atoms depend, and it is on this ground that their chemical valency has partly been determined. -
James Chadwick - England
Chadwick's own research focused on radioactivity. Chadwick repeated Fredic and Irene Joliot-Curie's experiments but with the goal of looking for a neutral particle. He looked for one with the same mass as a proton, but with zero charge. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than than the proton's mass. Chadwick's investigation in 1932 proofs the existence of the neutron.
