History of the Atomic Model

Democritus

400 BCE

Greek philosopher Democritus said that there was a particle so small it couldn't be cut. He said that if you cut something in half over and over again, you'd get to the point where you can't cut it in half anymore, which was the atom.

John Dalton

1800

Around the 18th century, Dalton said that elements form into compounds, a chemical reaction is a rearrangement of the atoms, and that all atoms of an element are the same in properties and mass. He based this on the law of conservation of mass and the law of constant composition.

J.J Thomson

1897

He discovered the electron and put forth the “plum pudding model“ which was that the atoms were like a fruit cake, and the electrons were the pieces of fruit inside it. He discovered it by powering cathode rays and saw that the beam moved to a positive charge and away from a negative charge, which meant that the beam had something negative, which is the electron.

Robert A. Millikan

1908

He found out the mass of the atom by using xrays to ionize a chamber that could capture electrons, and the ones that were not captured fell because of gravity, which helped him know what is the mass of the atom.

Ernest Rutherford

1911

He discovered that there was also something positively charged in the center of the atom by firing alpha particles at a gold foil sheet. He expected them to bounce back because if there was only negatively charged particles they would, but some went right past, which meant that there was a positive particle there too near the center, which became known as the atomic nucleus.

Niels Bohr

1913

He said that electrons move in several orbits around the nucleus and can jump from one orbit to another without really “existing“ in between any and that the electrons in the orbits have different levels of energy.

Heisenberg and the Quantum Model

1925

Heisenberg was one of many scientists that developed the quantum model, which says that electrons don't really circle around the nucleus, and we can't say anything about their trajectory or position, only about where they're most likely to be.