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Democritus 400BC
One of the first atomic theorists was Democritus, a Greek philosopher who lived in the fifth century BC. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter "atomos," the Greek word -
Isaac Newton
Isaac Newton theorized a mechanical universe with small, solid masses in motion. -
Dalton
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 -
Dmitri Mendeleev
Dmitri Mendeleev created the periodic table. -
Thomson
J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. Therefore, he proposed a model of the atom which he likened to plum pudding. -
Rutherford
In 1911, Ernest Rutherford, a former student of J.J. Thomson, proved Thomson's plum pudding structure incorrect. Rutherford with the assistance of Ernest Marsden and Hans Geiger performed a series of experiments using alpha particles. Rutherford aimed alpha particles at solid substances such as gold foil and recorded the location of the alpha particle "strikes" on a fluorescent screen as they passed through the foil. -
Bohr
Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. -
Erwin Schrödinger
Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucl -
Chadwick
James Chadwick discovered neutrons, particles whose mass was close to that of a proton.
