Atoms and Molecules

Dipole Moment

A dipole moment occurs in any system in which there is a separation between the charges. Dipole moment can arise in ionic bonds as well as in covalent bonds. The dipole moments occur due to the difference in electronegativity. This electronegativity is between two chemically bonded atoms.

A bond dipole moment is a measure of the polarity between two atoms in a molecule. Bond dipole involves the concept of electric dipole moment. It is a measure of the separation of negative and positive charges in a system. The bond dipole moment is a vector quantity therefore it has both magnitude and direction both.

Dipole moment

                                                                                  Dipole moment

Important Facts about Dipole Moment

  1. The dipole moment of a single bond in a polyatomic molecule is the bond dipole moment and it is different from the dipole moment of the molecule as a whole.
  2. It is a vector quantity, i.e., it has magnitude as well as definite directions. Since dipole moment is a vector quantity, it can be zero as the two oppositely acting bond dipoles can cancel each other.
  3. By convention rules, the dipole moment is denoted by a small arrow with its tail on the negative centre and its head on the positive centre.
  4. In chemistry, the dipole moment is represented by a slight variation of the arrow symbol. The dipole moment is denoted by a cross on the positive centre and an arrowhead on the negative centre. This arrow shows the shift of electron density in the molecule.
  5. In the case of a polyatomic molecule, the dipole moment of the molecule is the vector sum of all bond dipoles in the molecule.

The formula of Dipole Moment

A dipole moment is the product of the magnitude of the charge and the distance between the centres of the positive and negative charges in a system. It is denoted by the Greek letter ‘\mu’.

Mathematically,

\(Dipole Moment (\mu) = Charge (Q) \times distance of separation (r)\)

The dipole moment is measured in Debye units. It is denoted by ‘D’. \(1 D = 3.33564 \times 106{-30}C.m\). Here C is Coulomb and m is a meter.

The bond dipole moment arises between two atoms of different electronegativities. It can be expressed as

\(\mu = \delta.d\)

Here, \(\mu\) is the bond dipole moment,

\(\delta\) is the magnitude of the partial charges \(\delta^+ and \delta^- \) and d is the distance between \(\delta^+ and \delta^-\).

The bond dipole moment \(\mu\) is also a vector quantity. The direction is parallel to the bond axis. In chemistry, the arrows that are drawn in order to symbolize dipole moments begin at the positive charge and end at the negative charge. When two atoms with 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 shown via the bond dipole moment.

Examples of Dipole Moment

  1. The dipole moment of \(BeF_2\)

The bond angle between the two beryllium-fluorine bonds in the beryllium fluoride molecule is \(180^o\). Fluorine is a more electronegative atom than Beryllium so it shifts the electron density towards itself. The two individual bond dipole moments cancel each other in a \(BeF_2\) molecule. As they are equal in magnitude but are opposite in direction the net dipole moment of a \(BeF_2\) molecule is zero.

  1. The dipole moment of water \(H_2O\)

In a water molecule, the electrons are localized around the oxygen atom as it is much more electronegative than the hydrogen atom. The presence of a lone pair of electrons in the oxygen atom causes the water molecule to have a bent shape as per the Valence shell electron pair repulsion theory. Thus, the individual bond dipole moments do not cancel each other out as in the case in the \(BeF_2\) molecule. The bond angle in a water \(H_2O\) molecule is \(104.5^o\). The individual bond moment of an oxygen-hydrogen bond in a water molecule is 1.5 D. The net dipole moment in a water \(H_2O\) molecule is 1.84 D.

  1. Dipole moment in \(BH_3\) and \(NH_3\)

In the boron trihydride \(BH_3\) molecule, the dipole moment is zero. The dipole moment of ammonia \(NH_3\) is 1.49D. Boron trihydride \(BH_3\) has a symmetrical structure and the 3 B – H bonds are at an angle of \(120^o\) to each other. The 3 bonds are present in a single plane, so dipole moments cancel each other and the net dipole moment equal to zero. On the other hand, \(NH_3\) has a pyramidal structure, with 3 N – H bonds and a lone pair on the nitrogen atom. The resultant dipole moment of \(NH_3\) is 1.49 D.

Uses of Dipole Moment

The dipole moment is used to find the polar nature of the bond. As the magnitude of dipole moment increases, more will be the polar nature of the bond in a molecule. Molecules with zero dipole moment are non-polar in nature, while molecules with dipole moment are said to be polar in nature.

The dipole moment is used to find the structure or shape of the molecules. Molecules with specific dipole moment values will not have a symmetrical structure as they can be bent or angular in shape. While molecules with zero dipole moment will have a symmetrical structure or shape.

The dipole movement is in use for calculating the percentage ionic character in a molecule.

The dipole moment is used for determining the symmetry of the molecules. The molecules with two or more polar bonds within them would not be symmetrical and thus possess some dipole moment. For Examples \(H_2O = 1.84 D\), \(CH_3Cl\) i.e., methyl chloride = 1.86 D. If similar atoms in the molecule are joined to the central atom the resultant dipole moment zero. Such molecules have symmetrical structures. Examples: \(CO_2\), \(CH_4\).

The dipole moment is in use to distinguishing between cis- and trans-isomers. Usually, isomer with a higher dipole moment is trans-isomer and isomer with lower dipole moment are cis-isomer.

The dipole moment is in use to distinguishing between ortho, meta and para isomers. Usually, para-isomer have dipole moment zero, while ortho-isomer a have dipole moment that is greater than that of meta-isomer.

FAQs on Dipole Moment

Question 1: How do you find the largest dipole moment?

Answer: When there is a difference in the electronegativity of two atoms in a molecule involved in a bond, a dipole moment arises. The larger the electronegativity difference between the two atoms in a molecule, the larger the dipole moment and polarity.

Question 2: Write about the dipole moment of \(CO_2\)?

Answer: Carbon dioxide \(CO_2\) has a linear geometry at the core with carbon and oxygen on both sides. Thou, oxygen is more electronegative than carbon, the carbon electron cloud is pushed to oxygen and both oxygens pull the electron cloud from both sides with the same tendency. So, the net effect is zero.

Question 3: Define dipole moment in short.

Answer: The dipole moment \(\mu\) is the calculation of the net molecular polarity at either end of the molecular dipole. It is the magnitude of the charge Q times the distance d between the charges. Dipolar moments show the division of charges in a molecule. The dipole moment is measured in Debye.

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