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450 BCE
Democritus
He was the first person to believe that matter is not infinitely divisible.
His ideas were criticized and no one believed him because Aristotle, who was very well respected, thought that it was not true and so everyone else believed him. https://www.youtube.com/watch?v=thnDxFdkzZs -
450 BCE
Aristotle
He thought that Democritus’ idea was wrong and everyone believed him. Aristotle didn’t believe that there could be empty space.
It took 2,000 years for people to revisit Democritus idea.
He believed that matter is made of the four elements, earth, fire, air and water. -
Antoine Lavoisier
Antoine Lavoisier was one of the first people to use analytical balance to monitor chemical reactions.
He is given credit for the Law of Conservation of Matter -
Joseph Proust
Proust is responsible for the concepts of the Law of Definite Proportions. (that basically that the same elements have the same proportions by mass no matter the sample size) https://www.youtube.com/watch?v=nvTB2cMbWU8 -
John Dalton
He preformed experiments that allowed him to refine and support his hypothesis. Dalton's Atomic Theory explains the Law of the Conversation of Mass. (basically is says that the conservation of mass in chemical reactions is a result of separation, combination, and rearrangement of atoms)
His theory was not accurate -
JJ Thomson's Model
The Plum pudding model
Theory: the majority of the atoms is positively charged and the electrons are embedded in the positive "core".
He used a Cathode Ray to support his idea of electrons. The negatively charged electrons were deflected towards the positive end of the magnet.
https://www.youtube.com/watch?v=JUJPyQtoB5E -
Rutherford's Model
He identified the nucleus. His theory was that electrons are randomly around it.
An experiment that he did was the gold foil experiment. He made a machine that would shoot a beam of alpha particle (positive atoms) through a sheet of gold. (why gold? it is easily made to be small and thin) But of the alpha particles went through, but some of them bounced back.
He made to observations from this experiment.
1) the atom is made of mostly empty space
2) the nucleus is small and positive -
Bohr's Model
This is the most famous and well know "picture" of an atom. It's used everywhere... even though it's wrong.
His theory included neutrons in the nucleus and electrons in a fixed orbit -
Electron Cloud Model
This is the electron model that is currently accepted.
Electrons are random, but they are in energy levels. There is nor fixed path that that they follow. They can go in any of the energy level.
