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Democritus 440 B.C.
Democritus was a Greek philosopher. He argued that the world consisted of an infinite number of atoms moving in an infinite void. These atoms are invisible and indivisible particles of matter that were ungenerated and indestructible. They differ from one another in size, shape, and position. Each thing in the world is a different combination of these atoms. Our world came about as the chance combination of atoms, and since there are an infinite number of atoms, innumerable other worlds also have -
300
Aristotle
Aristotle’s theory made a great generalization off all matter being made 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. Based on these beliefs fire would hold the characteristics of being dry and hot, water is wet and cold, air is hot and wet, while the earth is dry and cold. -
Dalton, John (1766-1844)
John Dalton was an English chemist, who proposed an atomic theory of matter that became a basic theory of modern chemistry. His theory, first presented in 1803, states that each chemical element is composed of its own kind of atoms, all with the same relative weight. It explained why a fixed weight of one substance always combines with a fixed weight of another substance in forming a compound. -
J.J Thomson (1856-1940)
In 1897 the British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge-to-mass ratio. -
Ernest Rutherford
In 1909, Ernest Rutherford observed that alpha particles from radioactive decays occasionally scatter at angles greater than 90°, which is physically impossible unless they are scattering off something more massive than themselves. This led Rutherford to deduce that the positive charge in an atom is concentrated into a small compact nucleus. -
Ernest Rutherford 1911
Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. -
Niels Bohr
In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). -
Erwin Schrodinger and Werner Heisenberg
Erwin Schrodinger and Werner Heisenberg explained the nature of electrons in the atom. For example, electrons do not travel in definite paths as Bohr suggested. In fact, the exact path of an electron cannot be predicted. According to the current theory, there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds.
