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Parmenides - 580 BC
Parmenides came up with the idea that everything was made up of building blocks. This concept helped lead to the further discovery of the atom. -
Democritus - 400 BC
He developed the theory of the atom. This was how we discovered what made up the universe. His model was a solid sphere. -
Aristotle - 350 BC
Since Aristotle was a well known teacher, no one agrued when he came up with his theory of the atom. He believed that all matter was made up of fire, water, earth, air. Out of these, he believed that atoms had qualities of dryness, hotness, coldness, and wetness. -
John Dalton - 1800
Dalton had four major concepts to the atomic model:
1. All elements are composed of indivisible atoms.
2. Atoms of the same element are the same. Atoms of different elements are different.
3. Atoms make up compounds.
4. Atoms are rearranged in chemical reactions. None are created or destroyed.
His model was called the Billard Ball Model because it was a solid sphere and many others adopted this model. -
Dmitri Mendeleev - 1871
Mendeleev was the person that created the first periodic table with 63 elements. The elements were sorted by their atomic masses and he left gaps because he knew that there would be more elements added later. His model was a solid sphere. -
J. J. Thomson - 1897
Thomson first discovered the electron's existence, but he did not understand the finer details of it. He theory of the atom was the Plum-Pudding Model which consisted of a ball that was positively charged with neagitve charged particles that were randomly placed. He made this discovery throught the "Cathode Ray Tub" experiment where he hooked a tube up to wires and saw the electrons complete the circuit. -
Robert Millikan - 1908
Millikan discovered the exact charge of the electron and its mass. He also was able to find the mass of the atom. In the end, he said that the electron was 1/2000s of an atom in mass. He also believed in the Plum-Pudding Model that J.J. Thomson had developed. -
Ernest Rutherford - 1911
Rutherford was the first to understand that there were positive particles called protons that made up the nucleus with electrons all around it. The nucleus was believed to be very tiny and very dense. He came up with this conclusion through the Gold Foil Experiment. During this experiment, alpha particles were shot at a gold foil and were expected to go straight through the foil. Instead, some of the particles were deflected right back to the source of origin while some passed straight through. -
Niels Bohr
Bohr believed that the electrons orbited in rings or energy levels around the nucleus and he believed that they could jump from level to level to emit energy. -
Chadwick - 1932
Chadwick was the first to discover the neutron particle. He noticed that when he added together the masses of each particle, they did not make sense. So he noticed that the neutron was responsible for some of the mass. His model was the same as Bohr's, but with neutrons in the nucleus with the protons. -
Manhattan Project - 1941
The Manhattan Project was nuclear research that was done during WW2 to create the atomic bomb. Scientists proved that separating the nucleus released energy powerful enough to create an explosion. They used John Dalton's model, but it was proved incorrect when they separated the atom. -
Cloud Model
The Cloud Model is the newest description of the atom. It basically describes how electrons move around in an unknown path while the nucleus, which consists of protons and neutrons, stays in the center.
