jan 1, 1913 - Niels Bohr

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-1885 - 1962, Denmark
-In 1913 Electrons move around the nucleus in fixed orbits (like planets around the sun). An electron in a particular orbit has constant energy. An electron can absorb energy and move to a higher energy orbit of larger radius. An excited electron can fall back to its original orbit by emitting energy as radiation. Electrons can only exist in certain discrete energy levels.
-Lyman Series (ultraviolet): corresponds to excited electrons falling back to the lowest energy level (smallest orbit) known as the ground state. Balmer Series (visible): corresponds to excited electrons falling from higher energy levels to the first excited level.

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