The earliest model of the atom was made by J. J. Thompson. This model consisted of positive charges (protons were not yet a term back then) evenly placed and electrons entrenched in the atom like raisins in a raisin bread. Some names to this model were the Raisin Bread Model or the Plum Pudding Model. This model turned out to be very inaccurate for it was later proved by Rutherford that protons had to be in a tight enclosed space (nucleus).

Later on Rutherford and Bohr made models that were more accurate then Thompson’s even though they still had many mistakes. Both Rutherford and Bohr state that the mass of the atom is centered in the tiny core which is charged positively. Electrons are charged negatively, they weigh a lot less than the core and rotate around the nucleus and the radius of the orbit is significantly larger than the size of the core.  When changing an orbit from higher to lower, the electrons radiate light.

The difference between their models is that in Rutherford’s model, the movement is described by classical physics (Newton’s law) like the movement of the planets around the sun. The electrons are held by electrical force like the planets are held by the force of gravity. An electron can loose or gain any energy (E.g.: Collisions with alpha particles). When an electron loses energy it can emit light but the frequency of light (energy of light) can change continuously, thus it does not explain discrete specter of light. According to classical physics electrical charge moving along the orbit must emit light, so the atom would quickly collapse.

In Bohr’s model, electron moves on stationary orbits with a discrete set of energy unique for each atom. While on the orbit the electron does not emit light. Electron can move from one orbit to another (e.g., due to collisions) but, unlike in Rutherford’s model, it changes its energy discretely. When going from an orbit with a high energy to an orbit with a low energy it can emit a burst of light which is called photon. The energy of light is discrete, so the Bohr model, unlike Rutherford’s model, explains the discrete atomic specter.

A more recent and very accurate model was made by Erwin Schrödinger. This model was called the cloud model. Schrödinger used the Bohr’s model and improved it. Schrödinger proposed that an electron has no specific location, it can be located any point of space with some probabiliuty. From this he formulated an equasion which describes this probability. He charted a map, using his probability function, of locations it moved to. This map is shown in the picture below. This map shows a cloud of dots located around the nucleus. The places in which the cloud is denser are the ones where an electron would go more often, the places in the cloud where it is less dense are the ones where it would visit less often.