The VSEPR model

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The valence-shell electron pair repulsion (VSEPR) model is used for predicting molecular shape. The primary assumption of the VSEPR model is that regions of enhanced electron density (i.e. bonding pairs, lone pairs and multiple bonds) take up positions as far apart as possible so that the repulsions between them are minimised.

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Thus, in a molecule EXn, there is a minimum energy arrangement for a given number of electron pairs. For example, in HgCl2 repulsions between the two electron pairs in the valence shell of Hg are minimised if the Cl-Hg-Cl unit is linear. In HgCl3- electron-electron repulsions are minimised if a trigonal planar arrangement of electron pairs (and thus Cl atoms) is adopted. The table below shows the minimum energy arrangements and ideal bond angles for EXn molecules (where n = 2- 8).

 

Formula EXn	Number of Electron pairs	Shape	Spatial Arrangement	Ideal Bond Angle (deg.)
EX2	2	Linear		180
EX3	3	Trigonal planar		120
EX4	4	Tetrahedral		109.5
EX5	5	Trigonal bipy­ramidal		Xax-E-Xeq = 90 Xeq-E-Xeq = 120
EX6	6	Octahedral		X1-E-X2 = 90
EX7	7	Pentagonal bipy­ramidal		Xax-E-Xeq = 90 Xeq-E-Xeq = 72
EX8	8	Square Antipris­matic		X1-E-X2 = 78 X1-E-X3 = 73