9 05 2012

Molecular shape: the basic principles

Positioning of the bonds, and the bond angles, that form an atom are determined by electron repulsion between pairs of electrons (lone or bonded pairs).

Tetrahedron shaped molecules

Tetrachloromethane (CCl4), otherwise known as carbon tetrachloride, is an example of this, and it has four electron pair bonds. The 4 chlorine atoms are at the corners of a tetrahedron with the carbon atom at its centre.


The angle between adjacent bonds in this molecule is 109° 28’, and there is no other shape in which it could exist with bond angles greater than this.


The maximum angle has already been reached due to repulsion between the 4 pairs of bonded electrons. They want to be as far apart as possible without breaking the structure, think of it like magnets, electrons are negative and don’t want to be together or near each other particularly, they are attracted to positive things like protons in the nucleus of an atom.

Here’s a rough dot cross diagram I did on a post-it, the dots represent Carbon’s 4 electrons in its outer shell/energy level, therefore valence of 4, the crosses obviously represent Chlorine’s outer electrons (we know this because of its electronic configuration, see Lesson 2: Atomic Structure, Isotopes and Electronic Configuration)