Polarity & Dipole moment
In homo-atomic molecules having same type of atoms are present .In them sharing of electrons occur the shared pair remain in the middle that is equal attraction by atoms towards shared pair.
- The hetro-atomic molecules (having different types of atoms in them) when sharing of electrons occur the shared pair do not lie in middle, it gets displaced towards more electronegative atom.
- So, it will acquire partial negative charge and other atom will acquire partial positive charge. Due to this the poles are developed and the bond formed is polar covalent bond.
Polarity is measured in terms of dipole moment that is:
Let us calculate the dipole moment of different molecules with different atomicity. Dipole moment tells us about the structure of molecule.
- If dipole moment is zero, than the molecule is non polar.
- If dipole moment is not zero, than the polar molecule.
Dipole moment and molecular structure
Let us consider:
- Diatomic molecule: Its structure is generally linear.
Example:- In HCl, dipole moment is : 1.07 D
In HBr, dipole moment is : 1.78 D
In Hl, dipole moment is : 0.79 D
- POLYATOMIC MOLECULE: Their dipole moment depends upon the structure .As we know, dipole moment is vector quantity .Therefore, they will get added.
In carbon dioxide:
If we compare dipole moment of Ammonia and Nitrogen tri-fluoride:
In Ammonia, the H being less electronegative than N. All the electrons are directed towards Nitrogen. As a result, the vectors are added to give the resultant dipole moment.
In case of NF3, F being more electronegative than N .All the electrons are directed in opposite directions towards F. As a result, vectors are subtracted. Due to this reason NH3 has more dipole moment than NF3.
Importance of dipole moment:
- It gives distinction between polar and non polar bonds.
- If dipole moment is 0, than non -polar.
- If not zero, than it is polar.
- It tells us about degree of polarity.
- More is the dipole moment, more is the polarity.
- It gives information about shapes of the molecules.
- It gives percentage of ionic character in covalent bond. As we know, all ionic bonds have covalent character likewise all covalent bonds have ionic character in them.
- In covalent H-Cl bond, the bond is covalent, but it has ionic character in it due to difference in their electro-negativities.
- If electronegativity difference is 1.7, than the bond have 50% ionic and 50% covalent character.
- If electronegativity difference is less than 1.7, than the bond is covalent.
- If electronegativity difference is more than 1.7, than it is ionic.
Formula to calculate ionic percentage: