Covalent Bond

COVALENT BOND It is the bond formed by the mutual sharing of electrons between the combining atoms of the same or different elements. LANGMUIR introduced the term covalent bond. The compounds, which are formed by the covalent bond, are called covalent compounds. Let us consider the formation of few covalent compounds: 1) Formation of Hydrogen molecules (H2) Each hydrogen atom has one valence electron in its outermost shell. When 2 hydrogen atoms approach each other then valence electron of each hydrogen atom form a shared pair of electron and attain a noble gas configuration [Helium (He) = 1s2]

2) Formation of Chlorine molecule (Cl2) Similarly 2 chlorine atoms combine with each other and form a chlorine molecule. Both the chlorine atoms have 7 electrons in their valence shell (Ne 3s23p5) and have one electron less than the nearest noble gas configuration i.e. Argon. So they contribute 1 electron each to form a shared pair between 2 atoms and attain stable electronic configuration i.e. Octet.

3) Formation of Hydrogen Chloride (HCl) In this, Hydrogen atom has only one electron and Chlorine atom has 7 electrons in their valence shell. So both the atoms will share one pair of electron between themselves and form a covalent bond after acquiring a nearest noble gas configuration.

Lewis Structure and Covalent Bond Lewis- Langmuir theory gave the Lewis structure of covalent molecules. The main conditions for writing the Lewis structures are as: 1) Each bond should be formed as a result of sharing of electron pair between the atoms. 2) Each combining atom should give 1 electron to the shared pair. 3) Both the combining atoms should attain noble gas configuration as a result of sharing of electron. 4) All the atoms in a formula should have 8 electrons in their valence shell except hydrogen atom. This hydrogen atom has 2 electrons and resembles the first noble gas i.e. Helium.

Multiple Covalent Bond When atoms share more than 1 electron pair then they form multiple covalent bond. When atoms share 2 electron pairs then the bond formed is double covalent bond, if the atoms share 3 electron pairs then it is called triple covalent bond.

For example: 1) Formation of Oxygen molecule (O2) – Oxygen atom has 6 electrons in it’s valence shell and each Oxygen atom has a tendency to share 2 electrons to complete it’s Octet and to get the configuration of Neon (Ne2,8), Noble gas. Then the 2 Oxygen atoms by sharing 4 electrons i.e. 2 bond pairs form a Double Covalent Bond.

2) Formation of nitrogen molecule (N2) Nitrogen atom has 5 electrons in it’s valence shell(2,5).Two nitrogen atoms, therefore contribute 3 electrons each to share 3 pairs of electrons and attain the configuration of neon (Ne 2,8). Thus the nitrogen atoms in N2 molecules are held by triple covalent bond.

Covalency It is defined as the number of electrons in which the atom of an element contributes for sharing while forming a covalent bond. For example: The covalency of Chlorine is 1 because 1 electron pair is shared, Oxygen is 2 because 2 electron pairs are shared and that of

Nitrogen is 3 because 3 electron pairs are shared.

Properties of Covalent Compound The compounds whose molecules consist of atoms held together by covalent bonds are known as covalent compounds. Their general properties are: 1) State of existence – These compounds exist as individual molecules since the intermolecular forces in these compounds are generally weak, so they exist in the form of liquid and gases. However, a few covalent compounds also exist in solid states (e.g. urea, sugar etc.) 2) Low boiling and melting point – Due to the presence of weak intermolecular forces between the molecules lesser amount of energy is required to break the bond. Hence these compounds generally have low boiling and melting point. 3) Solubility – These compounds are generally soluble in non- polar solvents like benzene, carbon tetrachloride etc. but insoluble in polarsolvents like water. 4) Non- conducting – These compounds are poor conductor of electricity in diffused and dissolved state because they don’t give ions in solutions. 5) Molecular reactions – These compounds are non-ionic in nature i.e. they don’t give ions in aqueous solutions. Their reactions involve the breaking of covalent bonds in reacting molecules and forming new covalent bond to give molecules of the products. These reactions are relatively slow because energy is required to break the covalent bond. 6) Directional character of the bond – Covalent bond is directional in nature because it is localized in between the nuclei of atoms.

Method of writing Lewis Structures The following steps should be adopted: 1) Calculate the total number of electrons required for writing the structure by adding the valence electrons of the combining atoms,e.g. in CO2 molecule there are 16 valence electrons,4 from carbon and 12 from two oxygen atoms (6 each). 2) For anions, each negative charge means addition of an electron to the valence electrons and for cations each positive charge means subtraction of electrons from the valence electrons e.g. O2- means 2 electrons are

added to the valence electrons of oxygen atom (6+2) and Li+ means 1 electron is removed from the valence electrons of lithium atom (3 -1). 3) Write the skeletal structure of the compound and distribute the total number of electrons as bonding shared pairs between the atoms in proportion to the total bonds. 4) The least electronegative atoms should occupy the central position in the molecule. For example in CO2 molecule carbon is least electronegative. Hence it will occupy central position. Hydrogen and Fluorine generally occupy the terminal position. 5) After distributing the shared pairs of electrons for single bond the remaining electron pairs can be used either for multiple bonds or as lone pairs. 6) Basically each bonded atom should have an octet of electrons

The above steps can be understood with the following example The Lewis formula for carbon monoxide (CO) 1) The total number of valence electrons are 4 (C)+6(O)=10 2) The skeletal structure of CO is written as CO 3) Drawing of a single bond between C and O and completing the octet on O, the remaining 2 electrons are the lone pairs on C

4) This does not complete octet on carbon atom and hence we have triple bond between C & O atoms.

This satisfies octet rule condition for both atoms.

Co-ordinate Covalent Bond It is another type of covalent bond in which both the electrons in the shared pair come from one atom. Such type of bond is formed between atoms or ions, one of which is deficient in at least 2 electrons while other atom already has stable noble gas configuration. The atom, which contributes the electron, is called the donor while the other atom, which shares the electron pair, is called acceptor. This bond is represented by an arrow → pointing from donor to the acceptor atom. This bond is also called dative bond, donor acceptor bond, semi-polar bond or co-ionic bond. Example – Ammonium ion (NH4+), Ozone molecule (O3), Hydronium ion (H3O+), Sulphuric acid (H2SO4) It is important to note that the co-ordinate bond once formed can’t be distinguished from covalent bond.

Explanation of a few examples of co-ordinate bond 1) Ozone molecule – O3, Oxygen molecule contains 2 oxygen atoms joined by double covalent bond so that octet of each oxygen atom is complete. If an atom of oxygen containing 6 electrons comes close to oxygen molecules then the new atom may share a lone pair of electron of one of the oxygen atoms of the molecule. This results in the formation of co-ordinate or dative bond.

2) Combination of ammonia and boron tri fluoride (BF3) – The combination takes place through co-ordinate bond

3) Sulphuric acid (H2SO4) – It has 2 co-ordinate bonds in its molecule. The bonds between sulphur atom and oxygen atom of 2-OH groups are the normal single bonds whereas the bonds between the sulphur atom and other 2 oxygen atoms are co-ordinate bonds.

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