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200
440 BC - Democritus
Thought you would be able tofind a particle that could not be split. He believed that this particle was called an atom. He said that all atoms are small, hard particles amd thought that they were made of a single material formed into different shapes and sizes. -
201
382 BC - Aristotle
He disagreed with Democritus's ideas. He believed that you would never end up with a particle that you could not cut. He had a really strong influence on people so they believed him for a long time even though he was actually wrong. -
Dalton
Dalton's atomic theory stated:
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.
Atoms join with other atoms to make new substances. -
Thomson
Thomson discovered that there are small particles inside the atom, which means that atoms can be divided into even smaller parts. He invented the Cathoe-ray. He then discovered that a positively charged plate attached the beam. The beam was made of negatively charged particles. These negativy charged particles are present in every kind of atom. These particles are called electrons. -
Rutherford 1909
He designed an experiment to study the parts of the atom. He aimed a beam of postivily charged particles at a thin sheet of gold.He put a special coating behind the sheet. The coating glowed when hit by the positivly charged particles. He discovered that most of the particles bounced back instead of going straight through. -
Rutherford 1911
Rutherford revised the atomic theory. He built a new model of the atom that had a tiny, extremely dense, positivly charged center. This center was called the nucleus. Because like charges repel, Rutherford said that positivly charged particles that passed close by the nucleus were pushed away because of the positive nucleus. -
Bohr
He proposed that electrons move around the nucleus in certain paths, or energy levels. He made a model where there are no paths between levels. But the electrons can jump from one path of a level to another. Bohr's model was a valuable tool in predicting some atomic behavior, but the atomic theory still had room for improvement. -
Schrodinger and Heisenberg
They further explained the nature of electrons in the atom. Electrons do not travel in definite paths like Bohr predicted. The path actually can't be predicted. They also discovered that there are regions called electron clouds where electrons are likely to be found.
