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400
Aristotle
He developed a theory that was based on the four elements. Aristotle’s theory made a great generalization of all matter of the four elements: fire, water, earth, and air. He also believed that there were four qualities to these elements: dryness, hotness, coldness, and moistness. -
460
Democritus
He created the first atomic model. His model of the atom depended on analogies from human senses. -
John Dalton
developed the first useful atomic theory of matter around 1803. In the course of his studies on meteorology, Dalton concluded that evaporated water exists in air as an independent gas. He wondered how water and air could occupy the same space at the same time, when obviously solid bodies can't. If the water and air were composed of discrete particles, Dalton reasoned, evaporation might be viewed as a mixing of water particles with air particles. He performed a series of experiments on mixtures -
William Conrad Roentgen
Discovered x-rays while using cathode-ray tubes. Found that x-rays could passthrough solid objects. -
Joseph John Thomas
he took the cathode ray experiments a step further by firstlyimproving Perrin’s version to more clearly prove cathode rays do carry negativecharges. With this , Thomson then went on to discover the electron through hisdemonstration of cathode rays responding to electrode fields just as negativelycharged particles would.- Thomson had figured out a way to determine the charge of the mass by usingboth an electric and magnetic field.- Used mutually perpendicular electric and magnetic fields to determine -
Robert Andrews Millikan
he determined the unit charge ofthe electron with his oil drop experiment tobe 1.60 x 10-19.- Millikan calculated the mass of each dropfrom its diameter, then observed the motionof the oil drops in a uniform field. Byanalyzing this motion, Millikan calculated theelectric force acting on each drop. He foundthat the charge of each oil drop was amultiply of 1.60 x 10-19.- Since others had already determined thecharge-mass ratio, Millikan could nowcalculate a reasonably accurate valuable forthe mass -
Ernest Rutherford
By 1909, he had shown that some radioactive elements, such as radium andthorium, emitted positively charged helium ions, which are also known as alphaparticles and when passed through a thing sheet of mica, a beam of alpha particleswill spread out. -
Niels Bohr
Corrected the critical flaw in Rutherford’s model.- Focuses on the quantization of energy of electrons.- Basic principles of Bohr’s model:1. Electrons can orbit the nucleus only at certain specific distancesfrom the nucleus. These distances are particular multiples of theradius of the smallest permitted orbit meaning the orbits in an atomare quantized.2. The electron’s distance from the nucleus determines both thekinetic and electric potential energy of an electron in orbit. -
Henry Mosely
Moseley was the first to clear and scientifically justify in 1913 the atomic number studying X-ray spectra of chemical elements. -
Werner Heisenberg
The evolution of today's atomic structure took nearly 40 to 50 years to finish. There were many milestones, like discovery of sub-atomic particles, finding out the charge on them, etc. Among them is the inclusion of Quantum mechanics. This provides a clear concept for the wave nature of electron and also brought across the concept of orbitals, or the three dimensional spaces. A scientist who played an important part in the merging of Quantum theory with atomic structure is Werner Heisenberg. H -
James Chawick
Chadwick's own research focused on radioactivity. In 1919 Rutherford had discovered the proton, a positively charged particle within the atom's nucleus. But they and other researchers were finding that the proton did not seem to be the only particle in the nucleus. As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example,
