Updated: Oct 23, 2019
Atomic theory is a scientific theory of the nature of matter, which states that all matter is composed of discrete units called atoms, the smallest units of an element. The idea of Atoms was first thought by the Greek philosopher Democritus. And it is from Ancient Greek that the word “Atom” comes from, which means indivisible, uncuttable. However, nowadays scientists have discovered that it is divisible as we see in Atomic bombs, that breaks up Uranium or Plutonium atoms.
Dalton's Billiard Ball Model:
The first Atomic model that was scientifically proved was created by John Dalton, he proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures.
J.J Thomson's "plum pudding" model:
Atoms were thought to be the smallest possible division of matter until 1897 when J.J. Thomson discovered the electron through his work on cathode rays. Thought his experiments he found out that the cathode rays could be deflected by electric fields, so he concluded that the rays were composed of negatively charged particles, which he called “corpuscles” (later on scientists would change its name to Electron). He found that electrons were much smaller and lighter than a Hydrogen atom, the smallest atom. Thus Atoms were not the smallest particles as scientists had previously thought.
Thomson suggested that atoms were divisible and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge, looking just like a Plum pudding.
Rutherford's Planetary model:
Ernest Rutherford, one of Thomson’s former students, discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center. In his experiment, he shot Alpha particles at thin sheets of metal and measured their deflection through the use of a fluorescent screen. He expected that it would be no significant deflection, but surprisingly, a small fraction of the alpha particles experienced heavy deflection. So Rutherford concluded that the positive charge of the atom must be concentrated in a very tiny volume to produce an electric field sufficiently intense to deflect the alpha particles so strongly. This led to the Rutherford’s Planetary model.
Bohr's Atomic model:
The scientist Niels Bohr proposed another model, for an electron, could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, its distance from the nucleus being proportional to its energy. So the electrons could only “orbit” in specific places called the energy levels.
Electron Cloud Model/Quantum Mechanics Model:
Louis de Broglie proposed that all moving particles behaved in a similar way of waves. Erwin Schrödinger studied this idea and described the electron in a wave function. This led to many conclusion that Bohr’s model could not explain. Many scientists after concluded that the electron behaved both as a wave and a particle. The modern model of the atom describes the positions of electrons in an atom in terms of probabilities. An Electron could then be found anywhere in the nucleus, but depending on its energy it can be found more frequently in specific places around the nucleus than others. Those places are called atomic orbitals.
https://en.wikipedia.org/wiki/Atomic_theory - 25/10/2017
https://www.nobeliefs.com/atom.htm - 25/10/2017
https://socratic.org/questions/what-are-the-6-models-of-the-atom - 25/10/2017
http://mdkchemistry.blogspot.se/2013/01/114-bohrs-model.html - 25/10/2017
https://www.sutori.com/item/untitled-c2cb - 25/10/2017
https://en.wikipedia.org/wiki/Geiger%E2%80%93Marsden_experiment - 25/10/2017