In chemistry and physics, an orbital is a mathematical expression that describes properties characteristic of no more than two electrons in the vicinity of an atomic nucleus or of a system of nuclei as in a molecule. An orbital is a three-dimensional description of the most likely location of an electron. Atomic orbitals are the basic building blocks of the atomic orbital model, a modern framework for visualizing the submicroscopic behavior of electrons in matter. There are four types of orbitals: s, p, d, and f. Within each shell of an atom, there are some combinations of orbitals. In the n=1 shell, you only find s orbitals, in the n=2 shell, you have s and p orbitals, in the n=3 shell, you have s, p, and d orbitals, and in the n=4 up shells, you find all four types of orbitals. The shape and size of an orbital can be determined from the square of the wave function. Atomic orbitals have distinctive shapes; all are centered on the atomic nucleus. The most commonly encountered orbitals in elementary quantum chemistry are the orbitals corresponding to the s, p, and d subshells. The order in which atomic orbitals are filled by electrons and the shapes of the orbitals are crucial factors in understanding the chemical behavior of atoms and their reactions.