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Period: 450 BCE to
Development of the Atomic Theory
The 'Atomic Theory of Matter' has changed drastically since 450BCE and is still ever evolving today. However, with the discover's of scientists such as Democritus, Dalton and Rutherford etc. we now have a greater understanding of how the world around us is amde and what it is made of. -
442 BCE
Democritus' Theory
Democritus believed There was a limit to how far you could divide matter. You would eventually end up with a piece of matter that could not be cut. He believd that an atom was a small indivisable particle. -
John Dalton's Theory
He added these theories to Democritus' origninal theory:
All substances are made up of atoms which 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 form different substances. -
J.J. Thompson's Theory
J.J. Thomson did an experiment with cathode ray tubes and discovered that atoms are made of smaller negatively-charged particles called electrons. He created the plum pudding model (simulating the nucleus of an atom), with there being negative and positive charged particles in an atom. -
Ernest Rutherford's Theory
Ernest believed that:
Most of the matter of the atom is found in a very small part of the atom. This is called the nucleus of the atom. It is very tiny and extremely dense (neutral and positively charged particles). Like charges repel so the nucleus must have a positive charge. If electrons have a negative charge they could not be in a positively charged nucleus. Electrons must surround the nucleus at a distance. Atoms are mostly empty space with a tiny, massive nucleus at the center. -
Bohr's Theory
Bohr proposed that electrons move in paths at certain distances around the nucleus.
Electrons can jump from a path on one level to a path on another level. -
Modern Theory of Atoms
It is now also believed that:
Electrons travel in regions called “electron clouds” You cannot predict exactly where an electron will be found. Electrons that are closer to the nucleus have less energy than those that are farther away from the nucleus.
