Upon completion of this module, you should be able to know how quantum mechanics determines the probable positions of electrons within the orbitals of an atom. R is the distance between the nucleus and electron and is typically measured in angstroms. All orbitals, except the 1s orbital, possess nodes. A node is a region or point in the orbital where the probability that an electron is located equals zero. For s orbitals, the number of nodes present is equal to n-1.