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VSEPR Theory

In molecules, atoms are arranged in a regular pattern with each other. These atoms are bonded with each other through chemical bonds; Ionic or covalent bonds. Ionic bonds are an electrostatic force of attraction between cations and anions which are oppositely charged ions. Covalent bonds are formed by equal sharing of electrons from the valence shell of elements. All elements try to get the Noble gas configuration by either losing or gaining electrons.

The chemical bonding of elements in the molecule can be explained with the help of different theories such as valence bond theory, valence shell electron pair repulsion theory and molecular orbital theory. Valence bond theory purposed the concept of hybridisation and resonance. Hybridisation explains the bonding between bonded atoms and their certain geometry. Resonance explains the stability of molecule with the help of delocalisation of electrons. 

Valence shell electron pair repulsion theory explains the stability of those molecules which cannot be explained by valence bond theory. It is based on the concept of loan pairs and bond pairs on the center atom. If there is any lone pair on the central atom, it will show deviation from the regular geometry which is based on the hybridisation. There are several examples which can be better explained by this theory but cann0t explained by VBT.

 
VSEPR model or VSEPR theory talks about the shape of a molecule depending upon the number of atoms or ions surrounding the central atom/ion.

Separate emphasis has been given to the bonding pairs and lone pairs surrounding the central atom. Detailed theory and its postulates, with examples are given below.
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VSEPR Theory Definition

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This theory is very useful in predicting the geometry or shape of a number of polyatomic molecules or ions of non-transition elements.

This theory was proposed for the first time by Sidgwick and Powell in 1940 and developed by Gillespie and Nyholm in 1957. According to this theory
“The shape of a given species (molecule or ion) depends on the number and nature of electron pairs surrounding the central atom/ion of the species.”

VSEPR Theory Examples

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1. Shape of molecules/ions with central atom surrounded by two atoms:


Type: AB2

Example of such molecule is BeF2. They have a linear shape. A is the central atom and B, the bonded atoms. Only sigma bonding is present in such molecules, with no lone pair on the central atom.

F----------- Be --------- F


2. Shape of molecules/ions where central atom is surrounded by three atoms/ions:


Type: AB3 (3bp) and AB2 (2 bp and 1 lp)

Example of such molecule is BF3, GaCl3 etc with three bonding electron pairs and SnCl2, PbCl2 etc. with two bonding pairs and one lone pair. They have trigonal planar or angular or V - shape.

 

3. Shape of molecules/ions whose central atom/ion has four electron pairs:


Types: AB4 (4bps) , AB3 (3 bps and 1 lp), AB2 (2BPs and 2lps) and AB(3lps and 1bp)

Example: AB4 -CH4, SiCl4, etc. They have a tetrahedral shape. The bond angle is 109.5o.

Shape of Methane

4. Shape of molecule of type: AB5 (5 bonding pairs)


Example: PCl5, PF5, etc. They have trigonal bi pyramidal shape.

Shape of AsF5
Shape of PCl5

VSEPR Theory Bond Angles

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The change in the magnitude of bond angle is due to the fact that lone pair - lone pair repulsion is greater than lone pair - bonding pair repulsion, which in turn is greater than the bonding pair- bonding pair repulsion.

(lp - lp) repulsion > (bp - lp) repulsion > (bp - bp) repulsion

More the number of lone pairs on the central atom, the greater will be the contraction caused in the angle between the bonding pairs. This fact is clear when we compare the bond angles in Methane, ammonia and water molecules.

Molecule Number of lone pair on central atom
Bond angle(and contraction in bond angle)
NH3 1 107.5o (109.5 - 107.5 = 2o)
H2O 2 105.5o (109.5 - 105.5o = 4o)
CH4
0 109.5o (109.5 - 109.5 = 0)

The comparison of bond angles of Ammonia and water with that of methane shows that each of H-O-H bond angles in water is decreased from tetrahedral angle of 109.5o to a greater extent than in ammonia. The greater decrease in case of water is explained as follows.

The valence-shell of O atom in water has four electron pairs as N- atom in ammonia molecule. Two of these are bonding pairs (bps) each of which is attracted by two nuclei (of H and O atoms) while the remaining electron pairs are lone pairs, each of which is attracted by only one nucleus(of O atom), since these lone pairs originate from O- atom only. 

Thus, we see that O-atom in water molecule has two lone pairs, while Nitrogen atom in ammonia has only one lone pair in it.

Vespr Theory Bond Angles

Thus, in water molecule, there are three types of repulsion
  • Lp- lp repulsion.
  • Bp - lp repulsion and
  • BP- bp repulsion
While in ammonia molecule, we have only two pairs of repulsion:
  • Lp - lp repulsion and
  • Bp - bp repulsion
Therefore, each of the H-O-H bond angles in water molecule is decreased from a tetrahedral angle to a greater extent than in ammonia molecule.

VSEPR Theory Shapes of Molecules

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Postulates of VSEPR theory gives the shape of various types of molecules/compounds.

The shape of the molecules as described by the theory are

1. Spatial arrangement of electron pairs around the central atom/ion of a given molecule/ion

  • The electrons already present in the valence shell of the central atom/ion of a given species(=a) plus the electrons acquired by the central/ion as a result of bonding with the other atoms (=b), i.e, (a+b)/2, gives the number of electron pairs present in the valence shell of the central atom/ion.
  • This theory assumes that these electron pairs occupy localized orbitals which they arrange themselves in space in such a way that they keep apart from one another as far as possible so that they may experience minimum electrostatic repulsion between them and hence may give minimum energy and maximum stability to the species.
According to this theory
  1. If a central atom is surrounded by two electron pairs, (one bonding and one anti-bonding or lone pair), the arrangement of them around the central atom will be such that these pairs are at a very far distance from each other. Thus, the arrangement would be linear (Diagonal) and the angle between them would be 180o.
  2. When the central metal atom is surrounded by three electron pairs (1 bonding and antibonding-2), these three are present in the form of a triangle to aid maximum distance between them. Thus, the spatial arrangement would be a Trigonal shape and the angle between them would be 120o for three bonding pairs.
  3. The central atom surrounded by four electron pairs (Bonding + antibonding = 4) will having a geometry of a regular tetrahedral, the four electron pairs as apart from each other as possible, and at a bond angle of 109.5o.
  4. If the central atom is surrounded by five electron pairs (Bonding + antibonding = 5), the electron pairs are directed towards the corner of a trigonal bi pyramidal geometry. If all the five electrons are bonding pairs, the bond angle between the two equatorial electron pairs is 120o while that between axial and equatorial electron pairs is 90o. The angle between the two axial electron pairs is 180o.
  5. For a central atom surrounded by six electron pairs, the electron pairs will be directed towards corners of a pentagonal bi pyramid, with the bond angle between two equatorial electron pairs at 72o while that between the equatorial and axial electron pairs is 90o. The angle between the two axial electron pairs is 180o.
The table of number of electron pairs and the shape is given below.

Number of electron pairs
Spatial arrangement/Geometry/angle
2 Linear-180
3 Trigonal planar- 120
4 Tetrahedral- 109.5
5 Trigonal bi pyramidal -120
6 Octahedral – 90, 90
7 Pentagonal bi pyramidal- 72, 90

2. Regular and Irregular geometry of molecule/ion

  • The electron pairs surrounding the central atom/ion are either only bonding pairs or, some of them are bonding pairs and the remaining are lone pairs.
  • If the central atom/ion is surrounded only by the bonding pairs, then the species has a regular geometry, i.e, there is no distortion in the shape of the species.
  • If however, the central atom/ion is surrounded by the bonding orbitals and lone pairs, the bond angle gets altered from the value expected for a particular geometry of the molecule or ion.
  • The change in the bond angle changes the geometry of the molecule too, thus distorting the shape of the molecule.
Thus, “Presence of one or more lone pairs on the central atom changes the magnitude of bond angle, which in turn alters the geometry of a molecule or ion.”

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