What is Dipole Moment?
A dipole moment arises in any system in which there is a separation of charge. They
can, therefore, arise in ionic bonds as well as in covalent bonds. Dipole moments
occur due to the difference in electronegativity between two chemically bonded
atoms.
A bond dipole moment is a measure of the polarity of a chemical bond between two
atoms in a molecule. It involves the concept of electric dipole moment, which is a
measure of the separation of negative and positive charges in a system.
The bond dipole moment is a vector quantity since it has both magnitude and
direction. An illustration describing the dipole moment that arises in an HCl
(hydrochloric acid) molecule is provided below.
It can be noted that the symbols 𝛿+ and 𝛿– represent the two electric charges that
arise in a molecule which are equal in magnitude but are of opposite signs. They are
separated by a set distance, which is commonly denoted by ‘d’.
Important Points
The dipole moment of a single bond in a polyatomic molecule is known as the
bond dipole moment and it is different from the dipole moment of the
molecule as a whole.
It is a vector quantity, i.e. it has magnitude as well as definite directions.
Being a vector quantity, it can also be zero as the two oppositely acting bond
dipoles can cancel each other.
By convention, it is denoted by a small arrow with its tail on the negative
center and its head on the positive center.
In chemistry, the dipole moment is represented by a slight variation of the
arrow symbol. It is denoted by a cross on the positive center and arrowhead
on the negative center. This arrow symbolizes the shift of electron density in
the molecule.
In the case of a polyatomic molecule, the dipole moment of the molecule is
the vector sum of the all present bond dipoles in the molecules
Dipole Moment Formula
A dipole moment is the product of the magnitude of the charge and the
distance between the centers of the positive and negative charges. It
is denoted by the Greek letter ‘µ’.
Mathematically,
Dipole Moment (µ) = Charge (Q) * distance of separation (r)
It is measured in Debye units denoted by ‘D’. 1 D = 3.33564 × 10 -
30
C.m, where C is Coulomb and m denotes a metre.
The bond dipole moment that arises in a chemical bond between two
atoms of different electronegativities can be expressed as follows:
μ = 𝛿.d
Where: μ is the bond dipole moment,
𝛿 is the magnitude of the partial charges 𝛿+ and 𝛿–,
And d is the distance between 𝛿+ and 𝛿–.
The bond dipole moment (μ) is also a vector quantity, whose direction
is parallel to the bond axis. In chemistry, the arrows that are drawn in
order to represent dipole moments begin at the positive charge and
end at the negative charge.
When two atoms of varying electronegativities interact, the electrons
tend to move from their initial positions to come closer to the more
electronegative atom. This movement of electrons can be represented
via the bond dipole mome
