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400 BCE
Democritus
Democritus started the idea that there may be a building block to the universe, after asking the question of if you could break a piece of chalk infinitely or if it would stop at some ultimate particle. -
Period: 400 BCE to
Discovery of atom to modern model
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John Dalton
Dalton created the first atomic theory being firstly that all matter is made of very small atoms, secondly, all atoms of a given element are identical, thirdly atoms cannot be created, destroyed, or subdivided, fourthly that in chemical reactions atoms combine with or separate from other atoms, and fifthly that in chemical reactions atoms that combine do so in simple whole-number ratios. -
JJ Thomas
Thomas discovered that particles are not indivisible, when an electric current runs through an atom it breaks down into two parts; one being a negatively charged particle and the other a positively charged particle. This lead to the creation of the pudding model which was essentially just John Dalton's model but with little dots all around inside of it. -
Ernest Rutherford
Through experiments, he believed that atoms consist of mostly empty space and that the positive charge in an atom is packed into one region of the atom deemed it the nucleus. While the nucleus part was correct the rest did not stand to be true, as the electrons remain in motion constantly but not in his model. -
Niels Bohr
Bohr determined that there are places in the atom where electrons can travel without losing energy, like how planets orbit the sun. This was put to the test and proven to be correct, the model consisted of two parts, a nucleus whose positive charge came tiny particle called protons, and one or more electrons orbiting the nucleus on the outside. -
James Chadwick (Jimmy Neutron)
Chadwick proposed that the reason for the mass difference in an atom is because there is a particle with no electric charge in the center of the nuclei. Chadwicks model consists of the nuclei having one or more protons and neutrons, outside of the nucleus are the electrons traveling in discrete orbits. -
Modern Theories
Modern chemists no longer use physical models and instead use mathematical models because it is impossible to accurately show an atom with a physical model due to the large number of discoveries in the 1900's which quickly led to physical models not being accurate. -
Different Models
