Bond Polarity

Molecular polarity is dependent on the presence of polar covalent bonds and the molecule's three-dimensional structure.

Learning Objective

Apply knowledge of bond polarity and molecular geometry to identify the dipole moment of molecules

Key Points

When non-identical atoms are covalently bonded, the electron pair will be attracted more strongly to the atom that has the higher electronegativity. This results in a polar covalent bond.
Polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment.
A polar molecule acts as an electric dipole that can interact with electric fields that are created artificially, or that arise from nearby ions or polar molecules.
The dipole moment $\mu$ that corresponds to an individual bond is given by the product of the quantity of charge, q, and the bond length r: $\mu = qr$ .

Terms

Bond polarity
A covalent bond is polar if one atom is more electronegative than its bonding partner, resulting in a net dipole moment between the two atoms.
Molecular polarity
A molecule is polar if it has a net dipole moment, which depends on the existence of polar covalent bonds and the molecule's three-dimensional structure or geometry.
dipole moment
A measure of the polarity of a covalent bond or of an entire molecule. It is the product of the charge on either pole of the dipole and the distance separating them.

Full Text

Bond vs Molecular Polarity

Polarity refers to the separation of charge that creates permanent positive and negative 'electric poles.' This concept can be applied in two contexts:

Bond polarity: when atoms from different elements are covalently bonded, the shared pair of electrons will be attracted more strongly to the atom with the higher electronegativity. As a result, the electrons will not be shared equally. Such bonds are said to be 'polar' and possess partial ionic character.
Molecular polarity: when an entire molecule, which can be made out of several covalent bonds, has a net polarity, with one end having a higher concentration of negative charge and another end having a surplus of positive charge. A polar molecule acts as an electric dipole which can interact with electric fields that are created artificially, or that arise from interactions with nearby ions or other polar molecules.

Dipole Moment

Dipoles are conventionally represented as arrows pointing in the direction of the negative end. The strength of a dipole's interaction with an electric field is given by the electric dipole moment of the bond or molecule. The dipole moment is calculated by evaluating the product of the magnitude of separated charge, q, and the bond length, r:

$\mu = q r$

In SI units, q is expressed in coulombs and r in meters, so μ has the dimensions of $C \cdot m$. If two charges of magnitude +1 and -1 are separated by a typical bond length of 100 pm, then:

$\mu = (1.6022 \times 10^{-19} C) \times (10^{-10} m) = 1.6 \times 10^{-29} C \cdot m = 4.8 D$

The Debye unit, D, is commonly used to express dipole moments.

Determining a Molecule's Dipole Moment

In molecules containing more than one polar bond, the molecular dipole moment is just the vector addition of the individual bond dipole moments. Being vectors, these can reinforce or cancel each other depending on the geometry of the molecule. Therefore, it is possible for molecules containing polar bonds to be nonpolar overall, as in the example of carbon dioxide.

Molecular dipole moment of carbon dioxide

The linear shape of the CO₂ molecule results in the canceling of the dipole moments of the two polar C=O bonds. The net, molecular dipole moment of CO₂ is therefore zero, and the molecule is nonpolar.

H₂O, by contrast, has a very large molecular dipole moment which results from the two polar H–O bonds forming an angle of 104.5° between them. The water molecule, therefore, is polar.

Dipole moment of a water molecule

Water has a very large dipole moment which results from the two polar H–O bonds oriented at an angle of 104.5° with respect to each other. The bond dipoles add up to create a molecular dipole (indicated by the green arrow).

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