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Pantoja Mary Rose

BSA 1-1 CS

Valence Shell Electron Pair Repulsion Theory

Valence shell electron pair repulsion theory, VSEPR , is a super-simple technique for predicting

the shape or geometry of atomic centres in small molecules and molecular ions:

Crucially, atomic centres with VSEPR determined geometry can be joined together into

molecular entities like cyclohexane and glucose:

This molecular building-block logic can be extended, enabling large biomolecular structures likeDNA to be modelled and understood:

The VSEPR Technique

Six or so steps are required to generate the VSEPR geometry of an atomic centre such as:



Carbon in methane, CH4

Nitrogen in ammonia, NH3

Xenon in xenon tetrafluoride, XeF4Iodine in the iodide difluoride ion, [IF2] –

First, determine the number of electrons in the outer (valence) shell about the central

atom (C, N, Xe, I, etc.):

Carbon, for example has four valence electrons, nitrogen 5, etc.

Second, find valency and number of electrons associated with the ligand X:

Third, construct a valid Lewis structure of the molecule in question showing all of the bonds and all of the lone pairs (nonbonded pairs) of electrons.

If the structure is a molecular ion, add one valence electron for each negative charge

and remove one valence electron for each positive charge.



Not all Lewis structures have eight electrons about the central atom A (as emphasised

by very simple Lewis octet theory). For example,

Sulfuric acid, H2SO4, has two monovalent OH functions and two doubly bonded

oxygens that behave as single ligands:

Phosphorus pentachloride, PCl5, has 10 electrons:

Fourth, determine the "total coordination number" of the central atom, where:

total coordination number =



number of electron pairs =

number of electrons in outer shell divided by 2

Methane, CH4, ammonia, NH3, the ammonium ion, [NH4]+ and the nitranion (amide

ion), [NH2] – , [above] all have eight electrons in the valence shell of the central atomand all have a total coordination number of 4.

Fifth, the overall geometry of the atomic centre is determined by the mutual repulsion

between the electron pairs of the total coordination number.

The effect can be replicated by holding 2, 3, 4, 5 or 6 balloons together:

2 Balloons give a linear geometry3 Balloons give a trigonal planar geometry

4 Balloons give a tetrahedral geometry

5 Balloons give a trigonal bipyramidal geometry6 Balloons give an octahedral geometry

Sixth, there two adjustments are required by the VSEPR method to find the geometry

of an atomic centre:

Lone pairs of electrons (nonbonded pairs) are taken into account in determining the

total coordination number and VSEPR geometry, but they are NOT used

when defining the geometry of an atomic centre, only the atoms are used:

For example, the oxygen of water has two bonded electron pairs (green) and twononbonded "lone" electron pairs (blue) giving a total VSEPR coordination number of

4.

But the geometry is defined by the relationship between the H-O-H atoms and water is said to be "bent" or "angular" shape of 104.5°.



Lone-pairs of electrons (blue) behave as if they are slightly bigger than bonded

electron pairs (green) and act to distort the geometry about the atomic centre so that

bond angles are slightly smaller than expected:

Methane, CH4, has a perfect tetrahedral bond angle of 109° 28' (109.47°), while the

H-N-H bond angle of ammonia, H3N:, is slightly less at 107°:

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