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In 440 BC, a Greek philosopher named Democritus came up with the conept of the atom.
He said that atoms were small, hard particles that could not be divided("uncuttable")
He also said that these atoms were always moving.
Theory based on observations gathered from many experiments from gethering elements.
Dalton's Atomic Theory:
All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed.
Atoms of the same element are exactly alike, and atoms of different elements are different.
Atoms join with other atoms to make new substances.
Thomson experimented with a Cathode- Ray Tube
Thomson called the negatively charged particles corpuscles.
We now call these negatively charged particles electrons.
Thomson's Plum Pudding Model:
His Plum Puding model showed a possible structure of the atom with the negative charges.
Rutherford discovered that the atom had a small, dense nucleus that contained positive charges.
Gold Foil experiment-aimed charged particles at a thin sheet of gold foil.
He was trying to see where positive particles would go.
Positvely charged particles that passed close to the nucleus were deflected away(like charges repel).
A particle that actually hit the nucleus would be deflected almost straight back.
Rutherford's "Planetary Model":
The atom is mostly empty space through which the electrons travel.
The atom has a small, dense, positivley charged nucleus.(Most of the mass is in the nucleus).
Electrons travel around the nucleus like planets around the sun.
In 1913, Niels Bohr determined that electrons traveled around the nucleus in definite paths.
He also discovered that electrons could jump from a path in one level to a path in another level.
Current Theory-Electron Clouds Schrodinger and Heisenberg
Schrodinger and Heisenberg explained that electrons do not travel in deifintite paths.
Electron paths cannot be predicted.
The current theory suggests that electrons are found in regions called electron clouds.