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435
Democritus
Democritus proposed that matter was formed of small pieces that couldn't be cut into smaller parts. He used the word atomos for these pieces. He made discovery in 435 B.C. -
Dalton
Dalton's theory stated that all elements consist of atoms that cannot be divided. All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses. An atom of one element cannot be changed into an atom of a different element by a chemical reaction. Compounds are formed when atoms of more than one element combine in a specific ratio. -
Thomson
Thomson's model of the atom stated that atoms contain negatively charged particles called electrons. Atoms must also contain a positive charge which balances the negative charge of the electrons. -
Rutherford
Rutherford was Thomson's former students, yet he found evidence that challenged Thomson's model. Through his model he suggested that the atom is mostly empty space but has a positive charge at its center. Like charges repel each other, so Rutherford inferred that an atom's positive charge must be packed within a small region in its center, called the nucleus. Any particle that was strongly deflected was repelled by a gold atom's nucleus. -
Bohr
Bohr was one of Rutherford's former students. He revised the atomic model again in 1913. He then suggested that electrons are found only in specific orbits around the nucleus. Each possible electron orbi in Bohr's model has a fixed energy -
Schrodinger
He used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. This model predicts the odds of where an electron will end up. -
Chadwick
He bombarded beryllium atoms with alpha particles when an unknown radiation was produced. This radiation consisted of particles which we now call nuetrons
