Dipole Moment

When electrons are shared unequally among atoms in a molecule, a dipole moment is formed. This happens when one atom is more electronegative than another, causing that atom to pull harder on the shared pair of electrons. The water molecule, which consists of one oxygen atom and two hydrogen atoms, is one of the most prevalent instances. Oxygen has a partial negative charge, while each hydrogen has a partial positive charge due to differences in electronegativity and lone electrons.

What is a dipole moment?

An electric dipole is formed when two electrical charges of opposing sign and equal magnitude are separated by a distance. The dipole moment (μ) is used to calculate the dipole’s size. The dipole moment is measured in Debye units, which are equal to the charge multiplied by the distance between the charges (1 Debye = 3.34×10-30cm).

When there is a charge separation in a system, a dipole moment occurs. As a result, they can be found in both ionic and covalent bonds. The difference in electronegativity between two chemically linked atoms causes dipole moments.

The bond dipole moment determines the polarity of a chemical bond between two atoms in a molecule. It uses the idea of electric dipole moment, which is a measure of how far negative and positive charges are separated in a system. H2O is an example of a polar molecule. The structure of H2O is bent (by VSEPR theory) because of the lone pair on oxygen, which means that the vectors encoding the dipole moment of each bond do not cancel each other out. As a result, water is polar.

Water’s dipole moment is depicted in Figure. In chemistry, the dipole moment is represented by an arrow that moves from positive to negative.

On both the molecule (net) dipole moment and the individual bond dipoles, the vector points from positive to negative. The bigger electronegativity gap between the two atoms, the more electronegative that bond is. A considerable difference in electronegativity is required to be called a polar bond. The dipole moment is directed in the direction of the total vector quantity of the bond electronegativities.

Important Points

• The bond dipole moment is the dipole moment of a single bond in a polyatomic molecule, as opposed to the overall dipole moment of the molecule.

• It’s a vector quantity, which means it has both magnitude and definite directions.

• Because the two oppositely acting bond dipoles can cancel one other out as a vector quantity, it can also be 0.

• A little arrow with its tail on the negative centre and its head on the positive centre is used to represent it.

• The dipole moment is symbolised in chemistry by a modest variant of the arrow sign. 

Dipole Moment Formula 

The dipole moment is the product of the charge’s magnitude and the distance between the positive and negative charge’s centre. It is denoted by the Greek letter ‘µ’.

Mathematically,

Dipole Moment (µ) = Charge (Q) * separation distance (r)

It is measured in Debye units, which are indicated by the letter ‘D.’ 1 D = 3.33564 10-30 C.m, where C stands for Coulomb and m for metre.

In a chemical bond between two atoms with differing electronegativities, the bond dipole moment can be represented as follows:

μ = 𝛿.d

Where, μ is the bond dipole moment,

𝛿 is the difference in magnitude between the partial charges 𝛿+ and 𝛿–,

The distance between 𝛿+ and 𝛿–  is denoted by d.

The bond dipole moment (μ) is a vector quantity whose direction is the same as that of the bond axis. The arrows used to indicate dipole moments in chemistry start at the positive charge and stop at the negative charge.

When two atoms with different electronegativities collide, the electrons tend to shift away from their original places in order to approach the more electronegative atom. The bond dipole moment can be used to describe the movement of electrons.

Examples of dipole moment 

Dipole moment of BeF2

The bond angle between the two beryllium-fluorine bonds in a beryllium fluoride molecule is 180°. Fluorine moves the electron density towards itself because it is the more electronegative atom. The individual bond dipole moments of a BeF2 molecule are depicted in the diagram below.

Because the two individual bond dipole moments in a BeF2 molecule are similar in magnitude but opposite in direction, they cancel each other out, as shown in the image above. As a result, a BeF2 molecule’s net dipole moment is zero.

Dipole moment of H2O (Water)

Because the oxygen atom is far more electronegative than the hydrogen atom, the electrons in a water molecule cluster around it. On the other hand, the presence of a single pair of electrons in the oxygen atom causes the water molecule to bend (as per the VSEPR theory). As a result, unlike in the BeF2 molecule, the bond dipole moments do not cancel out. The dipole moment in a water molecule is depicted in the diagram below.

A water molecule’s bond angle is 104.5°. An oxygen-hydrogen bond has a 1.5 D individual bond moment. A water molecule’s net dipole moment is found to be 1.84D.

Polarity and Structure of Molecules

The overall polarity of a molecule is determined by its shape and the polarity of its links. Depending on its structure, a molecule with polar bonds may or may not have any overall polarity. The simple definition of whether a complex molecule is polar or not is if the positive and negative charge centres of the molecule overlap. The molecule has no overall polarity if these centres are located at the same position in space (and is nonpolar). If a molecule is entirely symmetric, the dipole moment vectors on each molecule cancel out, resulting in a nonpolar molecule. The structure of a molecule can only be polar if it is not symmetric.

Figure: CO2 and H2O charge distributions. Negatively and positively signed regions are represented by blue and red colours, respectively.

CONCLUSION

The charge separation in a molecule is revealed by dipole moments. The difference in electronegativities of connected atoms determines the dipole moment. NaCl has the biggest dipole moment because it has an ionic bond (i.e. highest charge separation).

The bond dipole moment uses the concept of the electric dipole moment to determine the polarity of a chemical bond within a molecule. This happens whenever positive and negative charges separate due to uneven attraction between the two atoms for the bound electrons.