- 1 Polarity in covalent bonds(Dipole Moment)
Polarity in covalent bonds(Dipole Moment)
Hello children…..let us start with this topic -polarity in covalent bonds(Dipole Moment).
Children, you know that in a chemical bonding, an equal number of electron sharing takes place. So we can say, for covalent bonding both the atom, in which sharing is taking place, must have the same electronegativity. For the formation of a covalent bond, this is only an ideal situation.
When both the atoms have the value of the same or almost the same electronegativities, the bond which is formed between the atoms is a pure covalent bond.
However, the covalent bond formation is also possible when the combining atoms are dissimilar and they have different electronegativities. In such a case, the covalent bond is not pure and contains some ionic character, which is known as polarity. Now we will get this point (Polarity in covalent bonds) in detail.
What Is a Partial Ionic Character In a Covalent Bond?
We know that electronegativity is the measure of the “tendency of atoms to attract the shared paired of electrons toward itself ” in a covalent bond. When the covalent bond is formed between two similar atoms (same electronegativity), there is no pulling of electrons, and shared paired lie just in the middle of the nucleus of both the atoms, which means electron clouds distributed symmetrically around the atoms. We can say this is the pure case of covalent bonding. This is known as a NON-POLAR covalent bond.
When there is a formation of a covalent bond that takes place between two atoms having different electronegativities, the atom having a higher electronegativity will attract the shared pair of electrons to a greater extent in comparison to another atom.
As a result, the shared pair of electrons gets shifted towards the more electronegative atom. The electron cloud around both the atoms is now no longer symmetrical and gets distorted. The electron cloud is now more dense around the more electronegative atom.
Because the electron cloud unevenly distributed, the atom having shared pair of electrons (high electronegative) acquire a partially negative charge and another atom which has less electron cloud (less electronegative) acquires a partially positive charge. (shown in the picture below).
After acquiring a partial positive and negative charge, the covalent bond develops a partial ionic character. Such a bond is referred to as polar covalent bond or partially ionic covalent bond and the molecules possessing such bonds are known as polar covalent molecules or simply polar molecules.
For example- HF is a polar covalent molecule.
In between hydrogen and fluorine atoms, the sharing of one-one electron takes place because both are the seekers of one electron to get stability.
But it is noticeable that fluorine is in the seventh group whereas hydrogen is in the first group so there is a noticeable difference between the electronegativities between hydrogen and a fluorine atom. As we know that electronegativity increases as we move left to right in a period, so we can say that fluorine is more electronegative than hydrogen.
That’s why electron cloud is shifted towards fluorine atom, due to which it acquires slightly negative charge and hydrogen acquires a slightly positive charge. Thus the formation of a polar covalent bond takes place. (Polarity in covalent bonds(Dipole Moment)
Such molecules having partial positive and negative charges separated by a distance called bond distance and such molecules are also known as Dipole (it means two poles).
Ionic Character in a Covalent Bond-
It is clear that a covalent molecule is changing into a polar molecule (having two poles) due to the difference in electronegativities. And that molecule behaves like an ionic molecule. Now the question is, what is the extent of ionic character in that polar covalent molecule?
Actually, this extent depends upon the difference between the electronegativity of both atoms. The higher the difference higher the extent of ionic character (high polarity), and the lower the difference, means less the extent of ionic character (low polarity). (Polarity in covalent bonds(Dipole Moment)
If the electronegativity difference is 1.7, the covalent bond is assumed to have a 50 percent ionic character.
DIPOLE MOMENT(a measure of polarity)-
Polarity can be measured in terms of a physical quantity known as dipole moment. It is represented by a greek letter μ (mu). It is defined as-
“It is the product of the magnitude of the charge present or either of the two atoms are separated in the molecule.”
It is a vector quantity and works in the direction of joining the positively charged atom to the negatively charged atoms.
If +q and -q are the charge of two atoms and r is the distance between the atoms, then the dipole moment μ is given by
μ = q × r
The direction of the dipole moment is indicated by a crossed arrow pointing from a positive end to a negative end.
The partial charges are of the order of 10−10 e.s.u. while the distance between the atoms are usually of the order of 10-8 cm. Therefore the dipole moment of a covalent bond will be of the order of 10−18 e.s.u. cm(μ = q × r).
Therefore, a unit called Debye has been introduced to measure the dipole moment. It is equal to 10−18 e.s.u. cm and is represented by D.
Thus, 1 D = 1 × 10−18 e.s.u. cm.
The dipole moment of a particular covalent bond is referred to as bond dipole. The bond dipole and net dipole moment of a molecule can be measured experimentally.
The dipole moment not only indicates the degree of polarity of bonds but also provides important information regarding the structure of the molecule.
Dipole moment and molecular structure-
Since dipole moment is a vector quantity, the net dipole moment depends upon the resultant of the dipole moment of all the polar bonds present in polar covalent molecules. Therefore the geometry of a molecule plays an important role to determine the dipole moment. The following examples are-
1- Diatomic molecules:
In polar diatomic molecules, two atoms are linked together by a polar covalent bond. In such a case, the dipole moment of the molecule is the same as that of the polar covalent bond. For example, the dipole moment of HF molecules is 1.91 D while that of HCl molecule is 1.o3 D.
NOTE– The dipole moment of HF is greater than HCl because the electronegativity difference in HF is greater than HCl, as F is more electronegative than Cl.
The triatomic molecules are either linear or angular(bent). examples are BeF2 and CO2 are the examples of linear molecules.
In both cases the two polar bonds are oriented in the opposite direction at the angle of 180°, Therefore both the dipole moment act in opposite and cancel each other.
Although these molecules possess polar covalent bonds, yet they are regarded as non-polar molecules because the net values of their dipole moment are equal to zero.
In an angular triatomic molecule of the type AB2, the net dipole moment is not equal to zero. Such molecules possess a definite value of dipole moment which depends upon the magnitudes of the bond dipoles and the bond angle.
3-Tetra atomic molecules:
Molecules of the types AB3 may or may not possess a net dipole moment depending upon their geometries.
For example- both BF₃ and NH3 arw tetra atomic molecules and are of the type AB3 but the net dipole moment of BF₃ is zero while that of NH3 is 1.47 D.
In BF₃, each B-F bond has a bond dipole. The zero value of the dipole moment of BF₃ molecule suggests that the three fluorine atoms present in it must be at the vertices of an equilateral triangle with the boron atom at the centre.
In this geometry, the resultant of the two B-F bond dipoles cancel the third B-F bond dipole leading to the zero value of the dipole moment.
NH3 has a dipole moment means its geometry is different from that of BF₃. Ammonia is found to have a pyramidal structure. In ammonia, the lone pair makes a large contribution to the net dipole moment.
Therefore, the overall dipole moments of the NH3 molecule is the resultant of the bond moment of the three N-H bonds, and that due to the lone pair, that is why ammonia has a net value of the dipole moment.
Thus dipole moment is an important tool to decide the geometry of a molecule possessing bond dipoles. It has been largely used to determine the shapes and bond angles of a large number of molecules.
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