An atom is the smallest part of the elements that make up everything on Earth. Particles of energy make up an atom, and only nuclear reactions can further split an atom. A variety of different models have been used over the past decades to speculate on how an atom works and what particles it contains.
TL;DR (Too Long; Didn't Read)
While there were a couple primitive models for atoms, you're most likely to come across the Bohr and Electron Cloud models in the classroom.
Billard Ball Model
In the very beginning of the 1800s, John Dalton proposed that atoms were like tiny, hard billiard balls. His view of completely solid atoms seems like a very basic idea now, but in 1803 it was groundbreaking. The experts at Colorado State University say this theory was a major contribution to chemistry. He also proposed that all atoms of one element are identical, and that each element had a different type of atom.
Plum Pudding Model
J.J. Thompson's Plum Pudding Model introduced the idea of positive and negatives charges existing in atoms. According to Visionlearning, he used cathode ray tubes and positively charged plates to demonstrate the existence of negative particles named electrons. He hypothesized that an atom resembled a plum pudding, or a sphere filled with positively charge liquid and dotted with negative electrons.
Sciencing Video Vault
Solar System Model
The planetary or solar system model was developed by Niels Bohr, the experts at the University of Tennessee say. Despite its inaccuracies and having been developed in 1915, it is the most common model taught to children today. The Bohr model shows a clump of neutrons and protons clustered in the center to represent the nucleus. Crossing rings, dotted with electrons, surround the nucleus.
Electron Cloud Model
The electron cloud model is the most updated atomic model available, and it was developed in the 1920s. The Colorado State University website states that Erwin Schrodinger and Werner Heisenburg changed the specific rings of the Bohr model into clouds that surround the nucleus. Each cloud contains a certain number of electrons, but this model best reflects how difficult it is to pinpoint where each electron might be in relation to the nucleus.