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John Dalton's Atomic Theory
He proposed the Atomic Theory in 1803 which stated that (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). -
Niel Bohr's
The theory that electrons travel in discrete orbits around the atom's nucleus, with the chemical properties of an element being largely determined by the number of electrons in its outer orbit.
