The shell model of atomic structure is based on the notion that electrons in an atom occupy "shells" that can fill up, so only a certain number of electrons will fit in a given shell. G. N. Lewis found this idea useful in explaining the chemical properties of different elements. Lewis's shell model is consistent with the Bohr model of the atom in which electrons are thought of as orbiting the nucleus. The "shells" are three-dimensional counterparts of two-dimensional circular orbits with different radii. Although we now know that the Bohr model of the atom is not correct, the concept of shells is still sometimes used to describe the arrangement of electrons in atoms according to the Pauli exclusion principle.