Dipole Moments

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  • April 29, 2023
9th Chemistry Ch#04 Chemical Bonding Dipole Moments

Dipole Moments:

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The dipole moment,µ ,is defined as the product of the distance, d, separating charges of equal magnitude and opposite sign, and the magnitude of the charge, q.

µ = d × q 

Hydrogen Bonding (H-Bonding):

When hydrogen (H) is covalently bonded to a highly electronegative atom X (O, N, F), the shared electron pair is pulled so close to X that a strong dipole results.

Since the shared pair is removed farthest from the H atom , its nucleus (the proton) is practically exposed. The H atom at the positive end of a polar bond nearly stripped of its surrounding electrons, exerts a strong electrostatic attraction on the lone pair of electrons around X in a nearby molecule. Thus:

“The electrostatic attraction between an H atom covalently bonded to a highly electronegative atom X and a lone pair of electrons of X in another molecule, is called Hydrogen Bonding.” Hydrogen bond is represented by a dashed or dotted line.

Points to Remember:

  1. Only O, N and F, which have very high electronegativity and small atomic size, are capable of forming hydrogen bonds.
  2.  Hydrogen bonds are longer and much weaker than normal covalent bonds
  3. Hydrogen bonding results in long chains or clusters of a large number of ‘associated’ molecules like many tiny magnets.
  4. Like a covalent bond, a hydrogen bond has a preferred bonding direction. This is attributed to the fact that hydrogen bonding occurs through p orbitals which contain the lone pair of electrons on X atom. This implies that the atoms X–H…X will be in a straight line.

Examples of Hydrogen-Bonded Compounds:

Water, H2O:
In the H2O molecule, two hydrogen atoms are covalently bonded to the highly electronegative O atom. Here each H atom can hydrogen bond to the O atom of another molecule, thus forming large chains or clusters of water molecules.Each O atom still has an unshared electron pair which leads to hydrogen bonding with other water molecules. Thus liquid water, in fact, is made of clusters of a large number of molecules.
Ammonia, NH3:

In NH3 molecules, there are three H atoms covalently bonded to the highly electronegative N atom. Each H atom can hydrogen bond to the N atom of other molecules or other electron efficient atom of any other neighboring molecule.

Quiz

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