× – C m The dipole moment in case of BeF is zero. This is because the two equal bond dipoles point in opposite directions and cancel the effect of each other. In tetra-atomic molecule, for example in BF , the dipole moment is zero although the B – F bonds are oriented at an angle of o to one another, the three bond moments give a net sum of zero as the resultant of any two is equal and opposite to the third. Let us study an interesting case of NH and NF molecule.
Both the molecules have pyramidal shape with a lone pair of electrons on nitrogen atom. Although fluorine is more electronegative than nitrogen, the resultant Peter Debye, the Dutch chemist received Nobel prize in for his work on X-ray diffraction and dipole moments. The magnitude of the dipole moment is given in Debye units in order to honour him. dipole moment of NH ( .
× – C m) is greater than that of NF ( . × – C m). This is because, in case of NH the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the N – H bonds, whereas in NF the orbital dipole is in the direction opposite to the resultant dipole moment of the three N–F bonds. The orbital dipole because of lone pair decreases the effect of the resultant N – F bond moments, which results in the low dipole moment of NF as represented below : The smaller the size of the cation and the larger the size of the anion, the greater the covalent character of an ionic bond.
The greater the charge on the cation, the greater the covalent character of the ionic bond. For cations of the same size and charge, the one, with electronic configuration ( n - ) d n ns o , typical