-
492 BCE
Democritus
Democritus took a sea shell and continually broke it in half until he couldn’t and then used a mortar and pestle. When he got to what he believed was the smallest piece he called it an atom. Atom is Greek for ‘indivisible’. His atom theory was that everything is made of atoms that don’t get deleted and are eternal. -
John Dalton
He was experimenting on mixtures of gases to determine the effect of properties of individual gases with the mixture. After analyzing the data he realized particles must be different sizes. Dalton’s atomic theory was that all matter is made of atoms, which can’t be broken down or destroyed. He also thought that atoms of an element are identical and that compounds are formed by a combination of 2 or more different kinds of atoms. -
J. J. Thomson
Thomson was experimenting with gas discharge tubes and discovered the electron. He noticed a movement, which he called cathode rays, and saw they moved from the negative end of the tube to the positive. The negatively charged particles that moved the rays to the positive sides were electrons. His model is called the “Plum Pudding Model”. This is because he thought the electrons and protons were mixed throughout the atom like plums are in plum pudding. -
Ernest Rutherford
His experiment was shooting positively charged particles at gold foil to see where the particles would go. He saw that while most went through, which supported Thomson’s theory, some were deflected, which didn’t support Thomson. From this he developed the “Planetary Model”. Since the positive particles were repelled when they went to the middle he decided there was a mass in the middle that was positive (made of protons) and the rest of the atom was mostly empty space with a cloud of electrons. -
Niels Bohr
Bohr’s model is even more similar to the solar system than Rutherford’s. He also thought there was a positive nucleus with electrons around it. However his model shows them traveling in orbits around the nucleus controlled by electromagnetic forces than just a cloud. He also believed there were different levels to the orbits and that electrons could move between them by releasing losing or gaining energy. He thought Level 1 can hold 2 electrons, Level 2 - 8, Level 3 - 18, and Level 4 - 32.
