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500 BCE
Democritus
Democritus believed that "atomos" were the smallest pieces of matter, and if you kept cutting something into pieces, the smallest piece is indestructible. He stated that this smallest piece was part of everything in the world. -
500 BCE
Heraclitus
Heraclitus believed that somehow, all matter was composed of fire. He thought that the world was full of elements that constantly changed shape and form due to a pattern -
350 BCE
Aristole
Aristole believed that four elements, or basic substances, made everything that existed in the universe. These things were air, earth, water, and fire. He also thought that other matter was simply a mixture of these things. -
Dalton
Dalton believed that all matter was composed of atoms that could not be subdivided. He thought that reactions were simply just rearranging the atoms. Also, he stated that all atoms of one element would be identical. -
Dalton's Theory
John Dalton had 5 postulates that he based his theory on.
1. All matter is composed of tiny particles called atoms.
2. All atoms of a given element are identical (shape, size, etc.)
3. Atoms cannot be subdivided
4. Atoms of different elements combine to form compounds
5. In chemical reactions, atoms are combined, separated, or rearranged. -
Thomson
JJ Thomson was mostly known for his experiment in which he used cathode ray tubes to identify the charges inside an atom. A cathode ray tube is a tube, without any air, that is filled with high voltage rays. These rays emit light on the other side of the tube. Because of this model, Thomson was able to find the truth: atoms can be subdivided into protons and electrons. Thomson even found the size of electrons, based on how much the light bent. -
Thomson's Plum Pudding Model
Thomson created the "Plum Pudding Model" based off a popular English dessert. In the model, it is clear to see that there are many protons and electrons moving around. He decided that there must be protons in the model because something was needed to balance out the electrons. -
Rutherford's Experiment
Ernest Rutherford conducted a famous experiment in which he shot alpha particles at a piece of gold foil. Based on previous atomic models, he expected the alpha particles to go straight through the foil. However, some of the particles didn't get past the foil, which would make JJ Thomson's model inaccurate. Based on this, he learned that an atom must have a dense nucleus and a cloud where electrons were. -
Rutherford pt. 2
This changed the way we think of atoms today and made the most accurate theory of the atom. Rutherford's model would have looked more like a small circle (the nucleus) inside of a bigger circle (the cloud). The nucleus would have contained protons and neutrons while the cloud would have electrons. -
Bohr's Model
The main thing that Bohr discovered was through the colors of released light when electrons move (LES). Previous models lead Bohr to assume that electrons could give off any color, thus be able to move around freely. However, he soon discovered that electrons can only have a certain amount of energy (depending on the atom) and position. Based on this, Bohr created his model where electrons were placed on certain parts of the atom's cloud. This advanced the atomic theories a great deal. -
Schrodinger- Electron Cloud Model
Schrodinger had a few new ideas about the atom. These were: electrons don't orbit, we can't tell where electrons are when they're in the cloud, and electrons gain energy the further away they move from the nucleus. He then made a model that showed a detailed atom that pointed to where the electrons would most likely to be found. This is different from Bohr's model because instead of having a specific place where electrons were located, he had a cloud where electrons are "likely" to be found. -
Chadwick- Electron Cloud Model 2.0
Chadwick used what he knew from previous models and made a huge discovery. He knew that there must be a neutral part of an atom to make up some of the mass in the nucleus. He thereby discovered neutrons. His model was almost the same as Schrodinger's except for the new addition of the neutron. This discovery lead to an atomic model that is very similar to one that we use today.
