Shapes+of+molecules+and+ions

Valence Shell Electron Pair Repulsion Theory Covalent molecules have distinct shapes. The shape of a covalent molecule can be found using the **//Valence Shell Electron Pair Repulsion Theory (VSEPR)//**. According to this theory the repulsion between the valence electron pairs determines the shape of the atom. The valence shells are always arranged in such a way so that they are as far away from each other as possible. For example if there are two valence electrons pairs, they will repel each other so that the angle between them is 180 o. That way they are the furthest apart:



If there are three electron pairs then they will form a trigonal planar shape with an angle of 120 o between them.



If there are four then the molecule has a tetrahedral shape (in 3-D) with an angle of 109.5 o between them.



//Note:// in all of the above cases the electrons were all bonding electrons. The shape of the molecule would be the same if there were lone electron pairs, but the angle between the bonded pairs would be smaller as the //lone pairs// __repel__ the //bonded pairs// more strongly.

Another thing is whether there is a single, double or triple bond (where one, two and three electron pairs are shared respectively) they all act as if there was only one pair of electrons being shared.


 * The following table provides example of molecules with four electron pairs (bonding and lone pairs) and how they look: **

//(The black cloud-like parts represent a lone pair) //

Tetrahedral 109.5 o || Trigonal Pyramidal 107 o || Non-linear/bent/V-shaped 104 o ||
 * ~  Number of non-bonding electron pairs ||~  Example ||~  Shape and bond angle ||
 * <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> None || <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Methane (CH 4 ) || <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;">[[image:methane.png width="267" height="259"]]
 * <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> One || <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Ammonia || <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">[[image:ammonia.png]]
 * <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Two || <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Water || <span style="color: #000080; font-family: Tahoma,sans-serif; font-size: 14pt;"> [[image:Water.png]]

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">As you can see the presence of a lone pair causes the angle between the bonding electrons to decrease. And as you can see only the bonding pairs are taken into account when describing the shape of the molecule (so that although with the lone pairs waters molecule looks tetrahedral, when looking at the bonding electron pair, the molecule looks V-shaped).


 * <span style="color: #20c593; font-family: Tahoma,sans-serif; font-size: 14pt;">The following table shows shape of molecules with less than four electron pairs: **

<span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;">linear 180 o || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;">Trigonal Planar 120 o || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;">Non-linear (V-shaped) 117 <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: super;">o ||
 * ~ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> Number of electron pairs ||~ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> Number of non-bonding electron pairs ||~ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> Example ||~ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Shape and bond angle ||
 * <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Two || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> None || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Carbon Dioxide || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> [[image:carbon_dioxide.png]]
 * <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Three || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> None || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Boron Triflouride || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> [[image:Boron_triflouride.png width="214" height="257"]]
 * <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Three || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> one || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> Sulfur dioxide || <span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;"> [[image:Sulphur_dioxide.png]]


 * By end of this lesson you should be able to: **
 * ** predict the shape and bond angle of a molecule using the VSEPR theory **
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The photos of molecules (in the tables) on this page were generated using the Phet simulations available on [] (accessed 10/04/2012)