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200
440 BC- Democritus
He thought that there was a particle called an atom which could not be divided any further. He said that atoms were small, hard particles and were made of a single material formed into different shapes and sizes. -
201
382 BC- Aristotle
Aristotle did not agree with Democritus and had a strong influence on the people so they sided with Aristotle. Aristotle said that there was never a particle that could not be cut. -
John Dalton
Dalton wanted to know why elements combine in certain proportions to form water. He concluded from experiments that elements are a single atom. His atomic theory stated that all substances are made of atoms, atoms are small particles that cannot be created, divided, or destroyed, atoms of the same element are exactly alike and atoms of different elements are different, also atoms join with other atoms to make new substances. -
J.J Thomson
Thomson found a mistake in Dalton's theory. He discovered that there were particles inside the atom. This proved that the atom could be divided further. Thomson used a cathode-ray tube which discovered negatively charged particles called electrons. Thomson later proposed a new model of the atom where electrons were located throughout an atom. -
Ernest Rutherford
A student of J.J. Thomson decided to test Thomson's theory. He aimed a beam of positively charged particles at a gold foil with a special coating behind it that glowed when hit by the positively charged particles. This showed where the particles went after hitting the gold. His results showed that some of the particles passed right through the gold while others deflected or bounced back. -
Ernest Rutherford
Rutherford revised the atomic theory and made a new model which had a nucleus in the center. The positive particles were rebounded by the positive charges of the nucleus and any particle going toward the nucleus would be shot back in the direction it came. -
Niels Bohr
Bohr studied the way that atoms react to light. He proposed that electrons move around the nucleus in certain parths. Bohr made a new model where there were no parths between the levels of energy. His model helped predict some atomic behavior. -
Schrodinger and Heisenberg
These two scientists explained the nature of electrons in the atom. They proved that Bohr's theory is wrong and said electrons path cannot be predicted. There are only regions of the atom where electrons are likely to be found. These areas are called the electon clouds.
