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)




7 responses

10 05 2012

Hi, thanks for commenting on my blog.
I like the photos you have on this post, they make it easier to visualise what you are talking about. Good simple explanations

10 05 2012

Thanks! I try to keep things simple as I find some of my past chem teachers overcomplicated things unnecessarily

12 05 2012

That’s good. It’s about minimizing energy. It might be good to include some uses of the molecule. That’s a big gap in chemical education. I look at this molecule, see a tetrahedral geometry, with those electronegative halogens hanging out, ready to steal electron density.

13 05 2012

Yes it’s clever how they minimse their energy usage by forming in the most energy efficient shape. Nice suggestion thank you I will start including some uses or do an extra post 🙂

13 09 2012

good diagrams really helpful to understand

27 10 2012

Glad you found it helpful

23 03 2013

thanks mate…

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