Chemistry: Chemical Bonding: Disadvantages of Kossel՚s Theory and Covalent Bond (For CBSE, ICSE, IAS, NET, NRA 2022)

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Disadvantages of Kossel՚s Theory

  • It explains bonding for electropositive elements of Group 1 and 2 (highly electronegative elements)
  • It doesn՚t explain the formation of molecules like SO2 or O2.

Covalent Bond

  • Lewis proposed that bond formation is achieved by “sharing of a pair of electrons” between the two atoms.
  • Both the atoms contribute an electron each to this pair.
  • H. + H. H: H H – H, Here the shared pair of electrons contributes towards stability of both atoms. This is called as Covalent bond.

Lewis Structure

  • Lewis՚s structure is the convenient way to represent the bonding in simple molecules.
  • Here, each element is represented by Lewis՚s symbol or electron-dot structure.
  • Lewis՚s symbol consist of chemical symbol of the element with number of dots which represent the electrons in the valence shell.

E. g.

Lewis Structure

Example of the ionic bond formation in NaCl is represented as:

Iconic Bond Formation

The covalent bond formation in HCl is represented as:

Covalent Bond Formation

The formation of chlorine molecules from 2 chlorine atoms are represented as:

Formation of Chlorine Molecules
  • In the above cl atoms, both Cl atoms acquire the electronic configuration of argon.
  • By Octet rule, the oxygen molecules involve in sharing of 2 pairs of electrons between 2 oxygen atoms and it forms 8 electrons or octet in their valence shell.
  • The electrons involved in the bond formation are called as bonding electrons.
  • Bond pair is the pair of electrons and pairs of electrons that are not involved in the bonding process is called as lone pair.
  • The nature of electron pair plays an important role in determining the shapes of the molecules.

Coordinate Covalent Bond

  • Coordinate covalent are electron rich element which donates a pair of electrons to an electron deficient element, such electron donor-acceptor bonds.
  • The coordinate bond is usually represented by an arrow pointing from a donor atom to acceptor atom.
  • This bond is identical to covalent bond in both polarity and strength.
  • The covalent and coordinate covalent bonds are similar bonds which can be distinguished only in the way they are formed.

Characteristic Properties of Covalent Compound

  • Due to weak forces of interaction between the molecules, they have low melting and boiling points.
  • The covalent compound doesn՚t have ionic species, so they are poor conductors of electricity.
  • They are insoluble in water and dissolve in non-polar solvents such as benzene, carbon tetrachloride.

Polar Covalent Bond

  • The shared pair of electrons is attracted by nuclei of both the atoms.
  • In a molecule, the negative electron atom has greater attraction for shared pair of electrons.
  • In most cases, the shared electron pair lies more towards the atom with higher electro negativity.
  • The bond acquires polarity or partial ionic character because of the unequal sharing of the electrons pair.
  • The difference in the electro negativity may be high so that the electron pair is practically under influence of a single atom.

Bond Polarity and Dipole Moment

  • The covalent bond is formed by sharing of a pair of electrons by the two atoms where each one of which contributes one electron.
  • When two atoms have similar electro negativity, then the sharing of the two bonded atoms is equal.
  • H2, Cl2, O2, N2 are some the examples of this bond.
  • It is encountered when the atoms are of the same element, such bonds are called as pure covalent bond and it is non-polar in nature.
  • If the electro negativities of the two bonded atoms are different, then the shared pair of electrons lies more towards the electronegative atom Eg: chlorine.
  • The covalent bond in HCl is a polar covalent bond.
Bond Polarity and Dipole Moment

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