Valence Shell Electron Pair Repulsion Theory

VSEPR (Valence Shell Electron Pair Repulsion) Theory:

In a molecule the constituent atoms have definite positions relative to one another i.e., the molecules have a definite shape. The theories of bonding that we have discussed so far do not say anything about the shape of the molecules. A simple theory called VSEPR theory was put forth by Sidgwick and Powell in 1940 to explain the shapes of molecules. It was later refined and extended by Nyholm and Gillespie in1957. This theory focuses on the electron pairs present in the valence shell of the central atom of the molecule and can be stated in terms of two postulates:

POSTULATE 1

The electron pairs (both bonding and non-bonding) around the central atom in a molecule arrange themselves in space in such a way that they minimize their mutual repulsion. In other words, the chemical bonds in the molecule will be energetically most stable when they are as far apart from each other as possible. Let us take up some examples.

BeCl2 is one of the simple triatomic molecules. In this molecule, the central atom, beryllium has an electronic configuration of 1s2 2s2 . That is it has two electrons in its valence shell. In the process of covalent bond formation with two chlorine atoms two more electrons are contributed (one by each chlorine atom) to the valence shell. Thus there are a total of 4 valence electrons or two pairs of valence electrons. According to the postulate given above, these electron pairs would try to keep as far away as possible. It makes the two electron pairs to be at an angle of 180o which gives the molecule a linear shape.

POSTULATE 2

The repulsion of a lone pair of electrons for another lone pair is greater than that between a bond pair and a lone pair which in turn is greater than between two bond pairs. The order of repulsive force between different possibilities is as under.

lone pair - lone pair > lone pair - bond pair > bond pair - bond pair

COVALENT CHARACTER IN IONIC COMPOUNDS (FAJAN’S RULE)

Due to the polarization of anion by cation in ionic molecules or compounds, the covalent character in the compound is introduced. i.e the ionic character between the cation and the anion decreases.

When a cation possesses high polarizing power, it attracts the electron cloud of the anion towards itself more strongly. Thus this causes the overlap of electron charge clouds of cation and anion. This overlap of electron cloud introduces covalent characters. Therefore, the greater the polarizing power of a cation, the greater will be the amount of covalent character produced in the ionic molecules.

The condition which was mentioned by Fajan are called Fajan’s rule, and these conditions are described below.

The cation should have a high positive charge on it

  • Higher is the positive charge on the cation in given ionic molecules, higher will be its polarizing power, and hence greater will be the magnitude of covalent character produced in the ionic bond.

The cation should be small in size

  • Smaller is the cation in its size, greater is its polarizing power, and hence greater in the amount of covalent character produced in the ionic molecules.

The cation should have ns2p6d10 configuration

  • A cation having ns2p6d10 configuration has greater polarizing power than the cation having ns2p6 configuration. Greater polarizing power produces covalent character in the ionic bond.
  • Take the example of CuCl and NaCl. CuCl is covalent and NaCl is ionic. This can be explained on the basis of electronic configuration. Cu+ ion has 3s23p63d10 configuration which polarizes Clion to a greater extend than Na+ ion having 2ss2p6 configuration.

The anion should have high negative charge on it

  • The greater the magnitude of the negative charge on the anion, the more strongly it gets polarized by a given cation. Hence, the ionic bond induces covalent character we can generalize this as the higher the charge on an anion greater is the extent of the covalent character produced in the ionic bond.

The anion should large in size

  • The larger the size of an anion, the more strongly it is polarized by a given cation, and consequently the covalent character increases.

Application of Fajan’s rule

  • Fajan’s rule can be used to explain the properties such as covalent character, ionic character, ionic character, melting points, solubility, etc of the ionic compounds.
  • For example, The melting points of the NaCl, MgCl2, and AlCl3 can be compared by using fajan’s rule.
  • The magnitude of the positive charge on cation increases from +1 (Na+) to +3 (Al3+). Thus, the polarizing power of cations increases from Na+ to Al3+. Na+<Mg++<Al+++
  • Ionic character increases, melting point increases; since size of cation increases and size of anions is constant.

  • Covalent character increases, melting point decreases; since size of anions increases and size of cations is constant.

Valence Bond Theory (Modern Concept of Covalent Bond)

Consider two hydrogen atoms A and B approaching each other having nuclei  $_{N_A}$NA  and  $_{N_B}$NB  and electrons present in them are represented by  $_{e_A}$eA and  $_{e_B}$eB.When the two atoms are at a large distance from each other, there is no interaction between them. As these two atoms approach each other, new attractive and repulsive forces begin to operate.

Attractive forces arise between:
(a) Nucleus of one atom and its own electron i.e., NA - eA and NB - eB
(b) Nucleus of one atom and electron of other atom i.e., NA - eB,NB- eA.

Similarly repulsive forces arise between:

Attractive forces arise between:
(a) Nucleus of one atom and its own electron i.e., NA - eA and NB - eB
(b) Nucleus of one atom and electron of other atom i.e., NA - eB,NB- eA.

Similarly repulsive forces arise between:

(a) Electrons of two atoms like eA - eB

(b) Nuclei of two atoms N- NB

Attractive forces tend to bring the two atoms close to each other whereas repulsive forces tend to push them apart.

In hydrogen ,the magnitude of the attractive forces is more than that of repulsive forces. As a result, the potential energy of the system decreases and a molecule of hydrogen is formed .

As the atoms start coming closer to each other from infinite distance, they start interacting with each other and the system starts losing its energy as the forces of attraction exceed the forces of repulsion. But at a certain equilibrium distance ,the forces of repulsion are just balanced by the force of attraction and the energy of the system become minimum. The two hydrogen atoms are said to be bonded together to form a stable system i.e. a molecule.

In the formation of a strong bond, more energy should be released by the system. Lesser the amount of energy liberated, weaker will be bond formed and larger is the amount of energy liberated, stronger will be the bond formed.

The energy required to break one mole of bonds of the same kind is known as bond energy or bond dissociation energy.

Overlapping of Atomic Orbitals:

 When two atoms come close to each other there is an overlapping of atomic orbitals. This overlap may be positive, negative or zero depending upon the properties of overlapping of atomic orbitals. The various arrangements of s and p orbitals resulting in positive, negative and zero overlap are depicted in the given figure.

The distance between the two nuclei is called Bond length.

If we want to break the bond ,i.e. to separate the atoms, we have to supply the same amount of energy.

A stronger bond is that which requires greater energy for the separation of atoms.

Types of overlapping and nature of covalent bonds: The covalent bond may be classified into two types depending upon the types of overlapping: (i) Sigma (s) bond, and (ii) pi (p) bond

Sigma(s)bond: This type of covalent bond is formed by the end to end (hand-on) overlap of bonding orbitals along the internuclear axis. This is called the head on overlap or axial overlap. This can be formed by-

(a) s-s overlapping: In this case, there is an overlap of two half-filled s-orbitals along the internuclear axis.

(b) s-p overlapping: This type of overlap occurs between half-filled s-orbitals of one atom and halffilled p-orbitals of another atom.

(C) p-p overlapping: This type of overlap takes place between half-filled p-orbitals of the two approaching atoms.

 Pi (p) bond: In the formation of p bond, the atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis.