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James was an English Physicist in mid 1900s.
https://goo.gl/EteQdk

He redefined all that was known of chemistry with his discovery of the neutron. Being a physicist his expertise wasn't in chemistry. He came across the neutron while experimenting for the creation of the atomic bomb. Ironic he found the neutron because without the understanding of atoms and how they work (what it is composed of), the atomic bomb would've never been invented, and the atom itself wouldn't have been fully understood.

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Aside from the early philosophers, J.J. Thomson was an English physicist in late 1800s.

https://goo.gl/pm8nEK

He came up with the idea that an atom isn't a solid; or in other words that's its composed out of simpler things than just an atom. He conducted an experiment using a cathode tube (depicted above), and studied it's electric currents. In conclusion to his experiment, he found out that the cathode gave off ray's of negatively charged particles (what are now called electrons). To get this word out to the public in ways they understood, he developed the "Plum Pudding Model", (depicted below).  https://goo.gl/vVsu0r

This discovery unlocked a whole new array of knowledge in the world of chemistry.

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John Dalton was an English chemist in late 1700s, early 1800s. https://goo.gl/92QRRv
He is the man who developed the first ever evidence the world has of an atom. He thought that the atom was:

hard

invisible

and, indestructible

He also had the idea that atoms of the same element are composed of the same thing. He made a conclusion based on his knowledge, that the joining of two different atoms created things called compounds: i.e. H20, NaCl, and CO2. This information defined chemistry forever.

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In 1913, Henry Moseley discovered that all atoms of an element have the same amount of protons in the nucleus. He essentially discovered that elements differ based on their amount of protons. goo.gl/ggC8rq
goo.gl/qNaSp7

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In 1777, Antoine Lavoisier introduced a formula relating to the conservation of mass. The conservation of mass stated that during a reaction, matter is neither created or destroyed. Lavoisier also distinguished between an element and a compound, an element is a pure substance and a compund is two or more different pure substances.
goo.gl/PhCR6O
goo.gl/fYqrZD

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Today, the atomic theory is based off of wave quantum mechanics. This theory states that electrons do not move about in a definite path. It is not possible to figure out exactly where an electron is. The possible location is determined by the amount of energy. There is a small positively charged nucleus and enough electrons to make the atom neutral. Electrons are in clouds which is where they are likely to be found. Their location is determined by the amount of energy they obtain. Electrons with the lowest amount of energy are found in the energy level closest to the nucleus while electrons with the highest amount of energy are in the energy levels further away from the nucleus.
goo.gl/Aqfnl9

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Another well known philosopher and his companion Plato believed that the 4 elements,

Air

Water

Fire

Earth

were the only substances that could make different compounds. https://goo.gl/DRGfDM

Their ideas were wrong but no one knew that because in that time, Aristotle and Plato were some of the most credited Philosophers. It was Democritus who was right all along and people ignored him.

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In 1913, Niels Bohr said that electrons went in an orbit around the nucleus. He called this orbit energy levels. Bohr also introduced the Bohr Model which stated that atoms give off or absorb radiation when electrons switch levels in their orbit. Bohr also said that each electron belonged in a specific energy level and had a definite path around the nucleus.
goo.gl/FOIfJs

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In 400 B.C. this philosopher had an idea that everything in the universe was made out of particles. He called them "Atomos", the greek word meaning not to be cut. He believed these "Atomos" had these following characteristics

all different shapes and sizes

capable of fusing together

always in motion https://goo.gl/k23XjY

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In 1908 Ernerst Rutherford experiemented with a gun that shot bullets called alph particles. Rutherford took a foil of pure gold and shot the gun at it. Rutherford anticipated that the bullets would go right through the foil. However, some bullets bounced off of the foil. Rutherford knew that in order to bounce back, the bullets must have hit something hard and positively charged. Rutherford went on to discover the nucleus of an atom and reasoned that all positive particles are in the nucleus. https://goo.gl/G8csvD