In elemental atomic structure, the model of the atom has undergone several significant changes. At one point, the electrons were thought to occupy fixed positions around the nucleus of the atom. Then, the idea was put forth that the electrons move in a circular orbit around the nucleus. This atomic model was further modified that the electrons not only move, but are hard to affix to any one general location, they are able to "jump" from one level to another, and then back again.
Electron structure, traditionally speaking, seems to have an outer electron shell structure of 8 electons. This would give 4 pairs of electrons, with each electron spinning in different, opposing directions. The number "3" seems to be the designated halfway point. An atom's outer electron shell either has 3 "single" (not paired up) electrons to donate to another atom's outer electron shell, or it has room to accept up to 3 "single" slots to fill, thus completing it's outer electron shell.