‘Quantum mechanics’ shows how Bohr's old quantum theory was superseded by a newer theory of quantum mechanics. Bohr's quantum theory worked well in single electron sytems, but not in multi-electron systems. Quantum mechanics allowed the development of Schrödinger's equation, which could theoretically predict the determination of electron energy levels in any system. The advantage of quantum mechanics over quantum theory lies in its treatment of electrons as waves. This allowed Schrödinger to apply mathematical boundary conditions to his equation and quantise the energy levels of electrons. Further work by Heisenberg showed that electrons can be thought of as being spread around a shell rather than being in one place.