VSEPR is abbreviated as valence shell electron-pair repulsion theory. This theory discusses the arrangement of atoms or group of atoms around a central atom in a covalent compound and is determined completely by the repulsion between electron pairs in the valence shell of the central atom. This theory assumes that the molecule will take a shape such that electronic repulsion in the valence shell of that atom is minimized.
Deciding the shape of the molecule:
- Select the least electronegative atom as this atom will be the best to share its electrons with the other atoms in the molecule.
- Then we count the outer shell electrons in the central atom.
- Then we will count the electrons used by atoms (outside) to make bonds with the central atom.
- By combining these two, we get the result as Valence shell electron pair.
- The shape of the molecule is based on VSEP number.
|VSEP Number||Molecule Shape|
Now, we will discuss each shape in detail:
- In this type of molecule, we find two places in the valence shell of the central atom.
- They should be arranged in such a manner (pointing in opposite direction) such that repulsion can be minimized.
- Example: BeF2
- In this type of molecule, we find three molecules attached to a central atom.
- They are arranged in such a manner (toward the corners of an equilateral triangle) such that repulsion between the electrons can be minimized.
- Example: BF3
- In two-dimensional molecules, atoms lie in the same plane and if we place these conditions on methane, we will get a square planar geometry in which the bond angle between H-C-H is 900.
- Now, if we consider all these conditions for a three-dimensional molecule, we will get a tetrahedral molecule in which the bond angle between H-C-H is 109028’.
- Example: CH4
- Let’s take an example of PF5. Here, repulsion can be minimized by even distribution of electrons towards corner of a trigonal pyramid. In trigonal bipyramid, three positions lie along the equator of the molecule. The two positions lie along an axis perpendicular to the equatorial plane.
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