which severely limits the classically accessible region around r = 0. This region around r = 0 is, however, accessible to a quantum particle via tunnelling. The wave function χ in this region however, decreases rapidly according to the power law (χ~r l ), the damping being stronger for higher l values. For l = 0 there is no centrifugal barrier and the electron (s electron) can therefore approach the nucleus infinitesimally closely.