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1. John Dalton (1803)

   • John Dalton, a British chemist and physicist, developed a theory that matter is simply composed of atoms of different weights and is combined in ratios by weight. Also proposed that these atoms are spherical, and are in motion.

2. , J.J. Thomson (1897)

   • In 1897, 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. The negative electrons represented the raisins in the pudding and the dough contained the positive charge.

3. Ernest Rutherford (1911)

   • In 1911, Ernest Rutherford, a former student of J.J. Thomson, proved Thomson's plum pudding structure incorrect. Rutherford 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. To the experimenters’ amazement, although most of the alpha particles passed unaffected through the gold foil as expected, a small number of particles were deflected at an angle, and a few ricocheted straight back. Rutherford concluded that the atom consisted of a small, dense, positively charged nucleus in the center of the atom with negatively charged electrons surrounding it.

4. Niels Bohr (1913)

   • In 1913, Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. Bohr's theory could explain why atoms emitted light in fixed wavelengths.