Polar Covalent Bonds
Polar Covalent Bonds
** In the H2
or Cl2 molecule, the two electrons constituting the covalent bond
are equally shared by the two identical nuclei. Due to even distribution of (+)
and (–) charge, the two bonded atoms remain electrically neutral. Such a bond
is called nonpolar covalent bond.
** However, when
two different atoms are joined by a covalent bond as in HCl, the electron pair
is not shared equally.
** Due to a
greater attraction of one nucleus (Cl) for the electrons, the shared pair is
displaced towards it. This makes one end of the bond partially positive (δ+)
and the other partially negative (δ–).
** A covalent
bond in which electrons are shared unequally and the bonded atoms acquire a partial
positive and negative charge, is called a polar covalent bond.
** A molecule
having partial positive and negative charge separated by a distance is commonly
referred to as a Dipole (two poles). The dipole of a bond is indicated by an
arrow from positive to negative end with a crossed tail as shown above in HCl
molecule.
** Since two
atoms of different elements do not have exactly the same attraction for
electrons in a bond, all bonds between unlike atoms are polar to some extent.
The amount of polarity of a bond is determined by the difference of
electronegativity (or tendency to attract electrons) of the two bonded atoms.
The greater the difference of electronegativity between two atoms, greater the
polarity.
** A graph
showing the % age ionic character and difference in electronegativity between
the two atoms is shown in the following Figure:
** As a matter
of fact, if this difference is around 1.9 and 2.9, the bond is generally ionic,
meaning that one atom has gained complete control of the electron pair in the
bond.
** The
percentage ionic character of a bond can be calculated by using the equation
% age ionic
character = 16 [XA – XB] + 3.5 [XA – XB]2
This equation
was given by Hannay and Smith.
Solved Problem
Calculate the
percentage ionic character of C–Cl bond in CCl4 if the
electronegativities of C and Cl are 3.5 and 3.0 respectively?
Solution
Examples of Polar Covalent Bonds
** Water
molecule (H2O) contains two O–H covalent bonds. The
electronegativity of O is 3.5 and that of H is 2.1. Thus both the bonds are
polar and water has a polar molecule.
** In ammonia
molecule, there are three N–H bonds. The electronegativity of N is 3.0 and that
of H is 2.1. Therefore all the N–H bonds are polar and ammonia has a polar
molecule.
** The
electronegativity of fluorine (F) is 4.0 and that of H is 2.1. The difference
of electronegativities being very great, the molecule H–F has a strong dipole.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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