VSEPR theory summary PDF

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"What you need to know" - VSEPR theory...

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What you need to understand: Valence Shell Electron Pair Repulsion Theory The principle of minimising electron-electron repulsion by placing electron pairs as far apart as possible. The degrees of repulsion: lone pair/lone pair > bonding pair/ lone pair> bonding pair/bonding pair

The following is taken from the web site: http://www.chemistry-drills.com/VSEPR.php

The VSEPR Technique Six or so steps are required to generate the VSEPR geometry of an atomic centre: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, [NH 4]+ and the nitranion (amide ion),

[NH2]–, [above] all have eight electrons in the valence shell of the central atom and 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 geometry 3 Balloons give a trigonal planar geometry 4 Balloons give a tetrahedral geometry 5 Balloons give a trigonal bipyramidal geometry 6 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 two nonbonded "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°:

What you need to be able to do: Given a molecule you need to be able to construct the Lewis structure and determine the formal charges on each atom. Given this, you need then to be able to decide which is the most probable Lewis Structure. In addition, you need to be able to draw a Lewis structure for a molecule, decide if resonance structures are possible and draw all possible resonance structures. You need then to be able to, using VSEPR, decide on the most likely (stable) 3dimensional shape for the molecule. Excellent web sites are:http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/vsepr.html http://www.chem.purdue.edu/gchelp/vsepr/example222.html A search on “google” for VSEPR or “Valence Shell Electron Pair Repulsion Theory” will result in many useful hits. In addition, your text book is again a very useful resource....