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.
For more detailed information about shape of molecules, join Byju’s.