A molecule in which the valence sub-shell of p electrons of one of the atoms appears to have more than eight electrons. Examples of electron-rich compounds include phosphorus pentachloride, PCl5, and sulphur hexafluoride, SF6. The existence of such compounds is a challenge to theoretical chemists as they do not appear to fit into the paradigm that stability for atoms with a valence sub-shell of p electrons is achieved when that sub-shell has eight electrons. Explanations for these compounds have been found using molecular orbital theory and valence bond theory.