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1803 - John Dalton
In 1803 John Dalton experimented with multiple substances to test why elements combine at certain proportions based on mass to form compounds. His results showed that elements combine in certain proportions because they are made of single atoms. In 1803 Dalton published his atomic theory, which stated that all substances are made of atoms, 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 - J.J. Thomson
In 1897 Thomson discovered that Daltons theory was wrong and that there are actually small particles inside of an atom, He experimented by using a cathode ray tube. He discovered that a positively charged plate attracted the beam. His conclusion was that the beam was made of particles that have a negative charge and that these negatively charged particles are in all atoms (now known as electrons). Thomson also created the plum pudding model which showed electrons mixed throughout an atom. -
1909 - Ernest Rutherford
In 1909 Ernest Rutherford decided to test his former teacher's theory (J.J. Thomson). He designed an experiment to study the parts of an atom. He aimed a beam of small positively charged particles at a thin sheet of gold foil. He put a special coating behind the foil. The coating glowed when the beam hit it. Rutherford could see where the particle went after hitting the gold. Most of the particles passed through the foil like he expected but some were deflected and even bounced straight back. -
1911 - Ernest Rutherford
In 1911, Rutherford revised the atomic theory and created a new model of the atom. His example showed that the center of an atom is a tiny, very dense, positively charged part called the nucleus. Rutherford explained that positively charged particles that passed close by the nucleus were pushed away because like charges repel each other. Also, a particle that went straight at the nucleus would be sent straight back in the same direction it came form. -
1913 - Niels Bohr
In 1913 Niels Bohr proposed that electrons move around the nucleus in certain paths or energy levels. In his model there were no paths in between levels. But electrons can jump a path in one level to a path in another. -
1935 - Erwin Schrodinger and Werner Heisenberg
In the 20th century Erwin Schrodinger and Werner Heisenberg helped to further define the nature of electrons in atoms. They discovered that electrons didn't travel in certain patterns like Bohr had stated and that the exact pattern of an electron can't be predicted. Also, they helped in the discovery of electron clouds, which are areas inside the atom where electrons are most likely found. -
382 BC - Aristotle
Aristotle was a Greek philosopher who disagreed with Democritus's idea that you can end up with a particle that could not be cut. He had such a strong influence on peoples ideas that for a long time most people thought he was right. -
440 BC - Democritus
In 440 BC a Greek Philosopher named Democritus thought that you could end up with a particle that could not be cut. He called this unbreakable particle an atom. He said that all atoms are small and hard particles. He thought atoms were made of a single material formed into different shapes and sizes.
