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They discovered two radioactive elements, polonium and radium.
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Atomic History Timeline made with Timetoast's free interactive timeline making software.

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In 1913, Henry Moseley experimented wavelengths of X-rays and discovered that the same elements have the same number of protons, which is their atomic number. Also, different elements have a different number of protons/atomic number. (Link) (pic)

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Discovered the Neutron
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He discovered a orbit model of an atom. Electrons can only have certain energy levels.
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Bohr's Model of the Atom, Niels Bohr Atomic Theory | Chemistry@TutorVista.com

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Neils Bohr gave a new arrangement of electrons in the atom called as Bohr's Model of an Atom. Learn about bohr model of the atom and niels bohr atomic theory with the help of our free online tutors.

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In the 1900's, scientists debated whether electrons move in waves or they are particles. Scientists eventually compromised that electrons are both. They are particles that move in waves. This is called the Quantum Mechanical Theory. They observed that the nucleus is tiny, and that electrons form a cloud that has a lot of empty space in it. (notes) (pic)

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Ernest Rutherford discovered the nucleus in 1908 when he shot alpha particles through a thin sheet of gold, and the bullets went in different directions, and sometimes bounced back. He realized that a positive charge must be causing the alpha particles to repel, so he said that there is a nucleus with a positive charge in the center of the atom. (notes) (Video) (pic)

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He discovered electrons and also discovered that atoms can be split. This led him to develop the Plum Pudding Model. The model shows that electrons are surrounding negative and positive charges. This was the best model of an atom before the nucleus was discovered.
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John Dalton created a theory in the 1800's that supported Democritus' theory that all things are made of atoms. He believed that the atom was an indivisible hard ball, and that the atoms of the same element are exactly alike. (notes) (pic)

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Democritus was a Greek philosopher that discovered atoms in 400 BC. He thought atoms were small hard particles that were different shapes and sizes. Aristotle did not believe Democritus. He thought that all substances are made up of fire, water, air, and earth.
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No, this is not a Greek equivalent of a baseball. It is what Democritus envisioned an atom would be. He suggested that atoms were small invisible particles that are indivisible by nature....