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400
Democritus
(400 BC) believed that atoms were indestructible and indivisble. -
John Dalton
English chemist and school teacher:
1. all elements are composed of atoms
2. atoms of the same elements are identical
3. atoms of different elements can physically mix together or can chemically combine
4. chemical reactions occur when atoms are separated, joined, and rearranged. -
Wilhem Roentgen
A German physicist who was first to observe x rays -
JJ Thompson
He determined the charge-to-mass ratio of an atom and pictures electrons embedded in a sphere with posistive charges. -
Rutherford Model
Ernest Rutherford finds that atoms have a dense, small, positively charged nucleus. Electrons also move around the nucleus. -
Bohr Model
In Niels Bohr's model, the electron moves in a circular orbit at a certain distance from the nucleus. -
Louis de Broglie
He proposed that moving particles such as electrons have some properties of waves. After a few years, experimental evidence supports the idea. -
Electron Cloud Model
Erwin Schrodinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model. -
James Chadwick
An English physicist confirms that neutrons exists and they have no charge. Atomic nuclei contain neutrons and positively charged protons.