Covalent Bond

When two or more atoms of the same element or atoms of different elements having similar electro negativities react, the transfer of electrons does not occur. In these instances, the atoms achieve inert gas (noble gas) structure by sharing of electrons. Thus, the atoms complete their outer most shell by means of sharing of unpaired electrons, and a covalent bond is formed. In covalent bond each atom has to contribute equal number of unpaired electrons. The shared pair of electrons which links the atoms in a molecule is known as covalent bond.

In covalent bond the shared electron pair is commonly expressed by single short line () For example, the halogens (chlorine atoms) posses an electronic configuration in which there are seven electrons in their outer most shell, and lacking only one electron in order to attain the structure of an inert gas. This structure may be attained by the halogens molecule (chlorine molecule) when both atoms share one electron for Bond formation

5.5.1 Single, Double and Triple Covalent Bonds:

  1. Single Covalent Bond: In single covalent bond only one pair of electrons is shared by the bonded atoms, in which each atom has to share one electron. This type of bond is represented by single short line (). For example, in the formation of (H2) and hydrogen chloride (HCl) molecules, only one pair of electrons is shared.
  2. Double Covalent Bond: In double covalent bond only two pairs of electrons are shared by the bonded atoms, and each atom has to share two unpaired electrons. This type of bond is represented by two short lines (=) as shown in the molecules of oxygen (O2) and carbon dioxide (CO2).
  3. Triple Covalent Bond: In triple covalent bond only three pairs of electrons are shared between the bonded atoms, and each atom has to share three unpaired electrons. This type of bond is denoted by three short lines (=) as shown in the molecules of nitrogen (N2) and ethyne (C2H2).

5.5.2 Characteristics of Covalent Compounds:

  1. Compounds with covalent bonds are usually made up of discrete units (molecules) with a weak inter molecular forces.
  2. In the solid state, there are weak Vander wall forces between the molecules. Hence covalent compounds are often gases, liquids or soft solids with low melting points. In few cases, three dimensional covalent structures are formed rather than discrete units, hence diamond and silica (SiO2) are covalent but are very hard and. have high melting points. Usually covalent compounds have low melting and boiling points.
  3. They are insulators because they do not conduct electricity.
  4. Covalent compounds are usually insoluble in polar solvents like water, but soluble in organic solvents like benzene, ether, carbon tetra chloride etc.

5.5.3 Electro negativity: (E. N.)

If the covalent bond is formed between two like atoms, that molecule is called non-polar-because the electron pair is shared equally between the two atoms, as in case of (H-H), (Cl-Cl), (0=0) and (N==N) molecules. However, if the covalent bond is formed between the two dissimilar atoms as in hydrogen chloride (H-Cl) molecule, the attraction for electron pair, would not be equal, one atom will attract more than the other. Hence the electron pair will be displaced from the central position and reaches near to the chlorine atom.

This power of an atom to attract the shared pair of electrons towards, itself, is known as electro negativity.

Consequently, the chlorine atom being more electronegative tends to be partially negative and hydrogen atom would be partially positive.

These values are based upon an arbitrary scale, in which fluorine is given an arbitrary standard value of electro negativity as 4.0. It is the most electronegative element. The electro negativity values of other elements are compared with that of fluorine. Note that the non metals have higher electro negativity values than the metals. Fluorine (F) has the highest electro negativity (4.0) and the metal cesium (Cs) has the lowest electro negativity (0.7) value.

5.4 Ionic Character in a Covalent Bond:

If covalent bond is formed between two like atoms e.g. in case (H-H), the hydrogen atoms are identical, and hence the shared pair of electrons is not disturbed from the centre. This molecule is called non-polar because it is electrically neutral as well as symmetrical.

If covalent bond is formed between dissimilar atoms e.g. in case of H-Cl molecule, the shared pair of electrons move closer to one of the both atoms. In H-Cl molecule chlorine is more electronegative, hence the shared pair of electrons, is drawn nearer to chlorine than hydrogen atom. This results in partial positive charge on H atom and partial negative charge on CI atom.

H+8 ------------ Cl-8

The covalent bond between H and CI in H-Cl is partially ionic or polar covalent bond, because of positive and negative charges or poles.

Covalent bonds are partially ionic, if they exist between two dissimilar atoms and heir ionic character depends upon the difference in electro negativities of bonded atom. 

  1. Non-Polar Covalent Bond: According to the scale of Linus Pauling, if the difference in the electro negativities of bonded atoms is zero, then the bond is pure covalent bond or non-polar bond. The molecule containing like atoms or the atoms of same electro negativities form a pure covalent bonds or non-polar bonds.
  2. Polar Covalent Bond: If the difference in the electro negativities of bonded atoms is up to 1.7 that bond is called polar covalent ¥or partially ionic in character. The elements of different electro negativities always form polar covalent bond.
  3. Electrovalent Bond: If the difference in the electro negativities of bonded atoms is more than 1.7 then that bond is purely ionic or electrovalent. The bond between sodium and chlorine in the common salt (NaCl) is" clearly ionic, because the difference in the electro negativities is 2.1 i.e. more than 1.7.

Polar covalent bond (HCl) and Ionic bond (NaCl)

 

 
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