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492 BCE
Democritus's Theory
Democritus proposed his atomic theory of the universe. He thought all material bodies are made up of indivisibly small “ atoms .” He basically believed that everything tin the universe consisted of smaller structures that he called atoms. -
340 BCE
Aristotle's Atomic Theory (kind of)
The Theory that Aristotle proposed was one that many people believed at the time. His model composed of four elements, Air, Water, Earth, Fire, and conditions those elements are found, Hot, Cold, Wet, and Dry -
John Dalton's Atomic Theory
Dalton put fourth the atomic theory of matter in 1803. He was a Physicist, Chemist, and meteorologist. Dalton was the first scientist to explain atoms in terms of weight, which he was soon able to use his discovery to calculate the relative size of an atom. -
Dimitri Mendeleev's Periodic Law
On March 6th Mendeleev gave a presentation about the dependence Between the Properties of the Atomic Weights of the Elements, to the Russian Chemical Society. That day was when he publicized his formulation of the periodic table. -
JJ Thomson's Discovery of fhe electron
Thomson's discovery occurred almost by accident when he was studying the properties of the cathode ray. -
Robert Millikan Determine's the Electron's Charge
His calculations were instrumental in demonstrating that electrons are essential, or indivisible, particles. He determined the electron's charge was negative, and later was honored with the Nobel Prize for Physics in 1923 for his studies of the electronic charge and the photoelectric effect. -
Marie Curie discovers Element's atoms cannot be broken down.
Curie Succeeded in discovering and isolating Radium which proved that the atoms of one element at least are not indivisible -
Rutherford's model of the Atom
Ernest Rutherford's famed Gold Foil Experiment of 1909 demonstrated that atoms were made up of a charged nucleus orbited by electrons. He published his findings in 1911 with a description of what he called the Rutherford model of the atom. -
Henry Moseley Invents the Atomic Number
He discovered the atomic number of each element using x-rays. His studies led to a more accurate organization of the periodic table. The relationship between x-ray frequency and atomic number is now called Moseley's Law. -
James Chadwick's Neuton Discovery
With Ernest Rutherford's help, Chadwick discovered the neutron by demonstrating a neutral particle with a mass the same as a proton through a reaction between gamma rays and a wax sample. -
Neils Bohr's Atomic model
Bohr was the first scientist to discover that electrons travel in separate orbits around the nucleus. He also noticed that the number of electrons in the outer orbit determines the properties of the outer orbit determine the properties of the element. Therefore, Bohr greatly improved the understanding of the atoms complexity and mechanics.
