In the 1920s, Erwin Schrödinger proposed that electrons travel in waves, which means their exact positions cannot be determined. He developed an equation to calculate the chances of an electron being in any given place. Using his equation, he identified regions around the nucleus, called orbitals, where electrons are most likely to be. Schrödinger’s work on orbitals is the basis of the modern model of the atom, which scientists call the quantum mechanical model. The modern model is also commonly called the electron cloud model. That’s because each orbital around the nucleus of the atom resembles a fuzzy cloud around the nucleus. The densest area of the cloud is where the electrons have the greatest chances of being. The nucleus is just a tiny part of the atom, but it contains virtually all of the atom's mass. The strong nuclear force holds together protons and neutrons in the nucleus and overcomes the electric force of repulsion between protons.