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Scientists discovered that electrons do not move in a definite path of orbit around the nucleus, and it is impossible to determine the exact location of an electron. Electrons with less energy are found closer to the nucleus while electrons with higher energy levels are found farther away from the nucleus.
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A Danish scientist named Niels Bohr in 1913 theorized that the negatively charged particles called electrons were located on different energy levels outside the nucleus. The electrons orbited the nucleus at different distances, like planets orbiting the sun.
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In 1908, an English physicist conducted experiments to understand and discover the structure of atoms. His experiment consisted of firing small, positively charged particles at a thin sheet of gold foil, roughly 2000 atoms thick. Most of the particles went right through the foil, but eventually one of the bullets bounced off of the foil. This disproved the plum pudding model, as in order for the positively charged particles to go through the foil, then the atoms could not be made of a positively charged substance with negatively charged particles throughout, because the positively charged substance would have repelled the bullets all of the time. Experimentation proved that atoms had to be made of some negative space, and included a nucleus with positive charges, which repelled the bullet.
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JJ Thomson, an English scientist in 1897, was the first to propose that atoms were made of even smaller parts. Through experimentation, Thomson found that some particles were negatively charged. For the neutral atoms to exist, there would need to be positively charged particles as well. Therefore, atoms must be made of smaller particles. He proposed that atoms were structured like plumb pudding- negatively charged particles in a positively charged substance, like raisins in pudding.
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An english chemist in the 1800s by the name of John Dalton performed a number of experiments that eventually provided proof for the theory of atomos. He figured out that all elements were made of atoms, which were invisible and indestructible particles. He also hypothesized that atoms of different elements were different, and that compounds were made of two different types of atoms bonding together. This theory became part of the foundation of modern chemistry.
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Democritus is a Greek philosopher who searched to define matter more than 2400 years ago. He questioned if matter could be divided into smaller and smaller pieces infinitely, or was there a point when it could no longer become any smaller. He hypothesized that at some point matter could not be divided any more and the smallest component of matter would be found. Democritus thought that atoms would be invisible to the naked eye, and that they would be round, hard balls that were made of the same materials. This theory was ignored for more than 2000 years because Aristotle, another philosopher, had a different theory that was more accepted at the time. His theory stated that all matter was made of either air, earth, fire, or water.
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