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1700's - Dalton
He was an English chemist and pysicist. He is best known for this pioneering work in modern character theory. John Dalton discovered that all substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. He also discovered Atoms of the same element are exactly alike, and atoms of different elements are different. And atoms join with other atoms to make new substances. -
1897 - Thompson
Thompson was a British scientist. He showed that there was a mistake in Dalton's theory. Thompson discovered that there are small particles inside the atom. This means that atoms can be divided into even smaller parts. He experimented with a cathode-ray tube. After learning that atoms contain electrons, Thompson proposed a new model of the atom. It is sometimes called the plum-pudding model, after a dessert that was popular in Thompson's day. -
1909 - Rutherford
Rutherford was a former student of Thompson who decided to test his theory. He did an experiment to study the parts of the atom. He aimed a beam of small, positively charged atoms at a sheet of gold foil. He put a coating behind the foil that glowed when hit by positvely charged particles. Then he could see where the particles went after they hit the gold. Rutherfod found that most of the particles went straight through the foil, but some of the particles turned to the side or bounced back. -
1911 - Rutherford
Rutherford revised the atomic theory. He made a new model of the atom. Rutherford suggested that in the center of the atom is a very dense, small, postively charged part which he called the nucleus. The nucleus is surrounded by positively charged particles and they are pushed away from the nucleus because they have a positive charge. He made a calculation that the diameter of the nucleus was 100,000 times smaller than the diameter of a gold atom. -
1913 - Bohr
Niels Bohr was a Danish scientist who worked with Rutherford and came up with the idea that electrons move around the nucleus in certain paths called energy levels. Electrons can jump from one engery level to another. This was important in predicting atomic behavior that scientists discovered. -
1925 - Schrodinger and Heisenberg
Ewrin Schrodinger was an Austrian physicist and Werner Heisenberg was a German physicist. They advanced the atomic theory by explaining the nature of electrons in the atom. Electrons do not travel in a definite pattern. The exact path of an electron cannot be predicted. They occupy an area called the elctron cloud. -
460 BC - Democritus
He formulated an atomic theory for the cosmos. He believed everything to be the result of natural laws. Everything is composed of atoms. In between atoms lies empty space. They are always in motion, there is an indefinite number and kinds of atoms which differ in shape and size. -
350 BC - Aristotle
Aristotle was a Greek philosopher. He believed that you would never end up with a particle that could not be cut. This was a disagreement with Democritus.
