"The behavior of a specific electron in an atom can be described by a mathematical expression called a wave equation—the same type of expression used to describe the motion of waves in a fluid. The solution to a wave equation is called a wave function, or orbital, and is denoted by the Greek letter psi (ψ). … What do orbitals look like? There are four different kinds of orbitals, denoted s, p, d,and f,each with a different shape. … The orbitals in an atom are organized into different electron shells, centered around the nucleus and having successively larger size and energy."