Valence Shell Electron Pair Repulsion Theory (VSEPR)

Valence Shell Electron Pair Repulsion (VSEPR) Theory and Lewis structures  are invaluable mdels that all chemists use as starting points for examining bonding and structure in molecular compounds. Bonding in the Lewis model is a result of the sharing of one or more electron pairs between adjacent atoms. This page contains a tutorial for constructing and visualizing VSEPR models. VSEPR can tell us much about the 3-dimensional structure of molecules. The 3-D structure of a molecule governs many important properties, such as polarity. The dipole moment of a molecule influences many of its physical properties, such as boiling point, freezing point, vapor pressure, and solubility. Also, the 3-D shapes of molecules play a crucial role in their reaction types and reaction rates.
Before we can begin to apply the VSEPR method to a molecule, we must know what atoms are present in the molecule and their connectivity, that is, which atoms are attached to which other atoms. In many cases, one atom serves as 'central atom' and the other atoms are attached to it. When looking at a molecular formula, the central atom is generally written first, with the ligands (attached atoms or groups) written next. For example, for the molecule CH4, C (carbon) is the central atom, and four H (hydrogen) atoms are attached to it. There are exceptions to this generalization though, such as H2O (water) in which each H is attached to the central oxygen atom.
Besides the atom-atom connectivity, we muxt also know the distribution of electrons within the molecule: how many electron pairs are shared between atoms, to form bonds, and how many pairs are unshared, belongin completley to specific atoms. The can be determined by drawing the Lewis structure. Keep in mind the following:
(i) H requires 2 electrons in its valence shell (thus it will only have one bond attached to
     it).
(ii) C,N,O,F always require 8 electrons in their valence shells (the "OCTET RULE").
(iii) heavier p-block elements may have more than 8 electrons ("expanded" octets).
(iv) Be,B,Al may have less than 8 electrons.
(v) formal charges should be kept as low as possible, ideally zero.
The last point may be used to determine the BEST Lewis structure when there are a number of possible electron arrangements.
The basic concept behind the VSEPR model is that the electron PAIRS around a certain atom will assume an arrangement which MINIMIZES the replusions between the negatively charged 'clouds' (electrostatic repulsions). The key is that ALL electron pairs about an atom must be considered, bonding pairs AND unshared pairs.
Let us start with a simple example, the molecule BF3. The best Lewis structure reveals the following arrangement of electron pairs:
 
Each F atom has an octet but B has only six electrons around it. Recall that B does not always achieve an octet of electrons. Note that the formal charges of all the atoms in this structure are zero. Now consider te electron pairs around B. There are 3 bonding pairs and NO unshared pairs, for a TOTAL of 3 PAIRS. The question is: what geometry will three negatively charged electron clouds use to minimize the electrostatic repulsions, but still remain anchored to the B atom? Simple electrostatics predicts a planar arrangement, with angles of 120o between any two adjacent B-F bonding pairs:
Since we cannot actually 'see' electron pairs (bonding or unshared), the shape of a molecule is determined by the postions of the nuclei. We see for BF3 that the three F atoms are all in the same plane as the central B atom. The shape is called 'TRIANGULAR PLANAR'. Here is a 3-D representation (Play with the image: LEFT mouse button down & drag ROTATES molecule; RIGHT mouse button down & drag ZOOMS in or out):
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