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440 BCE
Democritus
BCE-
He, along with his teacher Leucippus, thought that if you cut something continuously, eventually you will come down to the smallest piece that cannot be indivisible so he called the smallest piece an atomos. His earliest design included just a plain ball with no orbits, electrons, protons, or neutrons. -
340 BCE
Aristotle
Aristotle completely disagreed with Democritus and still stated that the only elements were earth, fire, water, and air so that's it, no further investigation needed. He also thought that atomos in a void couldn't be constant motion. -
John Dalton
Dalton was the next big important scientist after Democritus to contributed to the atomic theory, Dalton thought that atoms are round, hard, and smooth balls that cannot be broken into smaller pieces. His atomic theory states that all matter is made up of atoms that're indestructible and indivisible, all atoms of an element has the same properties and mass, compounds are formed when 2 or more different atoms are combined, and the rearrangement of atoms is called a chemical reaction. -
J.J. Thomson
Using a Cathode Ray, he was able to discover corpuscles (electrons) and it has less mass than protons and neutrons. In his "Plum Pudding" Model he found out that neutral atoms have an equal amount of positive and negative particles and proved Dalton's theory of atoms cannot be broken into smaller particles wrong. -
Hantaro Nagoaka
This Japanese scientist created the first version of an atom with an orbit that was based on Saturn's rings but it failed and he said that the rings are held there due to its massive orbit. His model contained a central nucleus with one orbit containing electrons. Although he failed, it helped other scientists, for example Bohr, in creating the correct updated version of the orbit containing electrons. -
Ernest Rutherford
Also known as the father of nuclear physics, he was the first person to draw a model featuring a nucleus. In his gold foil experiment, he shot tiny, positively charged particles at a golden foil, even though a majority of them passed through the atom, some did not and that some hit something Rutherford discovered, the nucleus which contains a majority of the atom's mass. In the end he concluded that atoms have mostly empty space with a few electrons. -
Niels Bohr
Bohr suggested that electrons are in orbit and they cannot move inbetween the orbits rather than jumping over to other orbits. These orbits have different levels with the first one having only 2 electrons, the next one has 8 electrons and the next has 18 and etc. The orbits are around the nucleus and his model name is the planetary model. -
Modern Atomic Theory
Upon Bohr's contribution to the atomic theory, we have finished what is now known as the modern atomic theory. The modern atomic theory includes that atoms of different types can fuse to form a compound, all matter is made from atoms, the number of protons in an atom is equal to the number of electrons, the number of neutrons can change, surrounding the nucleus are orbits containing electrons, and in the center of the atom lies the nucleus containing protons and neutrons.
