A nodal surface is defined as a surface where the amplitude of a standing wave changes sign (that is, where negative amplitudes become positive and vice versa) and the probability to find an electron equals zero. The term 'nodal surfaces' is used in the context of wave patterns for an electron in an atom of hydrogen. This depends on the principle quantum number. The principle quantum number is represented by the symbol n. When the electron as a standing wave is spread out, its motion around the nucleus can no longer be described in terms of orbits and its precise position at any given point of time cannot be described. The exact whereabouts of a spread-out electron can be found out by a probability function, which helps you to find it in a space known as an orbital. The probability is in proportion to the wave amplitude squared, which is known as electron density. All orbitals having the same principle quantity number n form a shell. Each orbital for the same n, when it is in the form of a standing wave, should have nodal surfaces of n-1.