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Joseph Priestley discovered oxygen in 1774 after conducting an experiment. He knew it to be colorless and a highly reactive gas. He named it "dephlogisticated air", but Antoine Lavoisier later named it "oxygen".
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Humphry Davy discovered that electricity has the power to react with chemicals and transform them. He was able to separate different elements.
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In the 19th century, John Dalton created this as a theory to help determine the composition of what different things were made out of. It is made up of three different "rules". The "law of conservation of mass" and the "law of constant composition" were both used to help establish the theory.
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In the 1800s, Amedeo Avogadro noticed that gases were made up of multiple atoms-molecules. He found out that the atoms in the elements form to make molecules. Something small creating something big.
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Friedrich Woehler, after discovering the chemical structure of a substance, he brings it up as an important part in the chemistry field. He came up with the shape and the matching carbon bonds. Then known that all organic substances had carbon in it. Chemical structure is important because everything now is built off of the template that Woehler had created back then.
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Robert Bunsen and Gustav Kirchhoff discovered that different colors can determine different substances. They built the spectroscope which has allowed scientists to do many things, one being space exploration.
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Dimitri Mendeleev had discovered the Periodic Table of Elements while trying to organize the elements into groups. Trying to put them into groups according to different descriptions, he then shortly realized that if he put them only according to atomic number, the rest would fall into place. He knew that there were elements yet to be discovered, so he left spaces for those ones. What he created changed the way everyone looks at the elements.
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Henri Becquerel was studying x-rays, which recently became famous. During that studying, he discovered the source of radioactive waves, uranium. Marie Curie and her husband, Pierre really pieced together the whole concept of radioactivity with all of their experiments.
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In the late 19th century, J.J. Thomson was experimenting with cathode ray tubes. To run observations, he chose to put 2 opposite electrically charged plates on each side. He saw that the cathode ray was pulled towards the positively charged plate and repelled by the negatively charged one (like how a magnet would do). He soon realized that the cathode rays were composed of negatively charged particles.
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Gilbert Lewis proposed that ionic bonds form due to the fact that atoms can share or accept valence electrons. This lead to the combining of atoms, which lead to chemical compounds.