Chemical Bonding (Part-3)

Subject: Inorganic Chemistry

Class: B.Sc - I (Hons) Paper Code/Name: Paper IB (Inorganic Chemistry)

Topic: Chemical Bonding (Part-3) Faculty Name: Dr. Rupali Gupta

College Affiliation: M. M. Mahila College, Ara

Date: 08/07/2020

Inert Pair Effect

Inert pair effect occurs when electrons are pulled closer to the nucleus, making them more stable and more difficult to ionize. An electron around the nucleus requires sufficient kinetic energy in order not to be pulled towards the nucleus.

This results in it having higher speeds, with a higher force acting on it by the nucleus.

The effects for the heavier elements are appreciable, as electrons travel closer to the speed of light, c.

The s-orbital electrons are more affected in this way since they have a greater penetrating power.

Thus Inert pair effect is defined as: “The non-participation of the two s electrons in bonding due to the high energy needed for unpairing them”. The inert pair theory was proposed by Sidgwick. He along with Powell accounted for the shapes of several molecules and correlated the shapes with some of their physical properties.

Example of Inert Pair Effect

• The inert pair effect among group 4 and group 5 elements. Sn2+ and Pb2+ and Sb3+ and Bi3+ which are the lower oxidation states of the elements are formed because of the inert pair effect.

• When the s electrons remain paired the oxidation state is lower than the characteristic oxidation state of the group.

Covalent Bond

• Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds.

• Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

• A covalent bond is formed by equal sharing of electrons from both

the participating atoms.

• The pair of electrons participating in this type of bonding is called shared pair or bonding pair.

• These bonds are also termed as molecular bonds.

How are Covalent Bond Formed?

• Elements having very high ionisation energies are incapable of transferring electrons and elements having very low electron affinity cannot take up electrons.

• The atoms of such elements tend to share their electrons with the atoms of other elements or with other atoms of the same element in a way that both the atoms obtain octet configuration in their respective valence shell and thus achieve stability.

• Such association through sharing of electron pairs among different or same kinds is known as Covalent Bond.

Formation of Covalent Bond

Covalent Bonding can be Achieved in two Ways: 1. Non Polar Covalent Bond: • Sharing of electrons between atoms of the same kind. • The shared pair of atoms have same electronegativity. • It occurs wherever the combining atoms have similar electron

affinity (diatomic elements). • Example: Formation of H2, Cl2, O2, etc.

2. Polar Covalent Bond: • Sharing of electrons between atoms of different kind. • The shared pair of atoms have difference in electronegativity. • More electronegative atom will have a stronger pull for

electrons. The electronegative difference between the atoms is greater than zero and less than 2.0. As a result, the shared pair of electrons will be closer to that atom.

• Example: Formation of CH4, H2O, NH3, etc.

Properties of Covalent Bond

If the normal valence of an atom is not satisfied by sharing a single electron pair between atoms, the atoms may share more than one electron pair between them. Some of the properties of covalent bonds are: • Covalent bonding does not result in the formation of new electrons.

The bond only pairs them.

• They are very powerful chemical bonds that exist between atoms.

• A covalent bond normally contains the energy of about ~80 kilocalories per mole (kcal/mol).

• Covalent bonds rarely break spontaneously after it is formed.

• Covalent bonds are directional where the atoms that are bonded showcase specific orientations relative to one another.

• Most compounds having covalent bonds exhibit relatively low melting points and boiling points.

• Compounds with covalent bonds usually have lower enthalpies of vaporization and fusion.

• Compounds formed by covalent bonding don’t conduct electricity due to the lack of free electrons.

• Covalent compounds are not soluble in water.

What is the Octet Rule?

• All atoms except noble gases have less than eight electrons in their valence shell.

• The valence shells of these atoms don’t have stable configurations.

• Therefore, they combine with each other or with other atoms to attain stable electronic configurations.

Therefore, “The tendency of atoms of various elements to attain stable configuration of eight electrons in their valence shells is the cause of Chemical combination” “The principle of attaining the maximum of eight electrons in the valence shell of atoms is called octet rule.”

• Lewis introduced simple symbols to denote the electrons present in the outer shell of atom known as the valence electrons.

• These symbols are known as Electron Dot Symbols and the structure of the compound is known as Lewis Dot Structure.

Dot structure of methane

Conditions for writing the Lewis dot structures

• Sharing of an electron pair between the atoms results in the formation

of covalent bonds. • During bond formation, each bond consists of two electrons which are

contributed by each one of the combining atoms. • By the mutual sharing of electrons, each atom attains octet

configuration in its valence shell.

• Electron dot structures of covalent molecules are written with respect to the octet rule.

• According to this rule, all the atoms in the molecule will have eight electrons in their valence shell except the Hydrogen atom.

• Hydrogen will have only two electrons because only two electrons complete its first shell to attain helium configuration.

• Thus the elements of group 17 such as Cl would share one electron to attain stable octet; the elements of group 16 such as O and S would share two electrons; the elements of group 15 would share three electrons and so on.

• For Example, the oxygen atom which has six electrons in its valence shell completes its octet by sharing its two electrons with two hydrogen atoms to form a water molecule.

Lewis Structure of Water Molecule

Types of Covalent Bonds

• Depending upon the number of shared electron pairs, the covalent bond can be classified into:

1. Single Covalent Bond 2. Double Covalent Bond 3. Triple Covalent Bond

1. Single Covalent Bond: • A single bond is formed when only one pair of the electron is

shared between the two participating atoms.

• It is represented by one dash (-).

• Although this form of covalent bond has a smaller density and is weaker than a double and triple bond, it is the most stable.

• For Example, HCl molecule has one Hydrogen atom with one valence electron and one Chlorine atom with seven valence electrons. In this case, a single bond is formed between hydrogen and chlorine by sharing one electron.

2. Double Covalent Bond: • A double bond is formed when two pairs of electrons are shared

between the two participating atoms.

• It is represented by two dashes (=).

• Double covalent bonds are much stronger than a single bond, but they are less stable.

• For example: Carbon dioxide molecule has one carbon atom with six valence electrons and two oxygen atom with four valence electrons.

• To complete its octet, carbon shares two of its valence electrons

with one oxygen atom and two with another oxygen atom.

• Each oxygen atom shares its two electrons with carbon and therefore there are two double bonds in CO2.

CO2 Molecule with Double Covalent bond

Ethylene Molecule: In ethylene, each carbon atom shares two of its valence electron with two hydrogen atoms and remaining two electrons with the other carbon atom. So there is a double bond between the carbon atoms.

Double Bond in Ethylene Molecule

3. Triple Covalent Bond: • A triple bond is formed when three pairs of electrons are shared

between the two participating atoms.

• Triple covalent bonds are represented by three dashes (≡) and are the least stable types of covalent bonds.

• For Example: In the formation of a nitrogen molecule, each nitrogen atoms having five valence electrons provides three electrons to form three electron pairs for sharing. Thus, a triple bond is formed between the two nitrogen atoms.

Nitrogen Molecule with Triple Bond

REFERENCES

(i) Basic Inorganic Chemistry, F. A Cotton, G. Wilkinson, and Paul L. Gaus, 3rd Edition

(1995), John Wiley & Sons, New York.

(ii) Concise Inorganic Chemistry, J. D. Lee, 5th Edition (1996), Chapman & Hall,

London.

(iii) Recent Aspects in Inorganic Chemistry, R. C. Aggarwal, 1st Edition (1987), Kitab

Mahal, Allahabad.

(iv) Principles of Inorganic Chemistry: Puri, Sharma and Kalia.

(v) Modern approach to fundamentals of inorganic chemistry by Biltu Singh, Kiran

Prakashan.

(vi) NCERT Chemistry, Class XI