11. Bonding in such clusters is often discussed in the context of polyhedral skeletal electron pair theory. 12. The electron pairs dominate the repulsions, and the tiny H atoms are insignificant in this respect. 13. When the electron pair moves towards the attacking reagent, it is termed as the + E effect. 14. High-energy gamma rays, upon striking the upper atmosphere, can create positron-electron pairs that move at great speeds. 15. The four electron pairs are spread so as to point roughly towards the apices of a tetrahedron. 16. This property makes them both excellent nucleophiles " and " lewis bases ( electron pair doners ). 17. However, they are Lewis acids, because they accept electron pairs from some Lewis bases, most notably base anhydrides. 18. The electron pairs can therefore be described as having a distorted tetrahedral geometry, but the molecule is pyramidal. 19. In short, a compounds is an acid because it donates a proton ( accepts an electron pair ). 20. If they are not, the remainder will form non-bonding electron pairs , usually known as " lone pairs ".
