How does the shape of the molecule affect the polarity?

Polarity occurs in covalent molecules. Covalent bonds are formed when two atoms of the same element or different elements share electrons so that each atom achieves its noble gas electron configuration. These covalent molecules can be either polar or non-polar .

This article explains, 1. What is polarity 2. How does molecular shape affect polarity 3. Examples

What is polarity

The polarity of a molecule defines its other physical properties such as melting point, boiling point , surface tension , vapor pressure, etc. Put simply, polarity occurs when the electron distribution in a molecule is asymmetrical. This leads to a net dipole moment in the molecule. One end of the molecule is negatively charged while the other receives a positive charge.

The main reason for the polarity of a molecule is the electronegativity of the two atoms involved in the covalent bond. In a covalent bond, two atoms come together to share a pair of electrons. The common electron pair belongs to both atoms. However, the attraction of the atoms to the electrons differs from element to element. For example, oxygen shows a stronger attraction to electrons than hydrogen. This is called electronegativity.

If the two atoms involved in bond formation have an electronegative difference of 0.4 <, the pair of electrons they share will be drawn towards the more electronegative atom. This results in a slight negative charge on the more electronegative atom while leaving a slight positive charge on the other. In such cases the molecule is considered to be polarized.

How does the shape of the molecule affect the polarity?

Figure 1: Hydrogen fluoride molecule

The strongly negative F in the HF molecule is slightly negatively charged, while the H atom is slightly positive. This leads to a net dipole moment in a molecule.

How does the shape of the molecule affect the polarity?

The polarization of a molecule strongly depends on the shape of the molecule. A diatomic molecule like HF mentioned above has no problem with shape. The net dipole moment is only due to the uneven distribution of electrons between the two atoms. However, when more than two atoms are involved in a bond, there are many complexities.

As an example, consider the water molecule, which is strongly polar.

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Figure 2: Water molecule

The water molecule has a curved shape. So when the two electron pairs that oxygen shares with two hydrogen atoms are pulled in the direction of oxygen, the net dipole moment results in the direction of the oxygen atom. There is no other force that cancels the resulting dipole moment. Therefore the water molecule is highly polar.

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Figure 3: Ammonia molecule

The ammonia molecule has a pyramid shape and the electronegative N atom attracts the electrons. The three NH bonds are not in the same plane; therefore the generated dipole moments are not canceled. This makes ammonia a polar molecule.

However, the dipole moments are sometimes canceled due to the shape of the molecules, making the molecule non-polar. Carbon dioxide is one such molecule.

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Figure 4: Carbon Dioxide Molecule

C and O atoms have a difference in electronegativity of 1.11, which means that the electrons are more strongly biased towards the O atom. However, the carbon dioxide molecule has a planar linear shape. All three atoms are on the same plane with C in the middle of two O atoms. The dipole moment of one CO bond cancels the other because they run in two opposite directions, making the carbon dioxide molecule non-polar. Although the electronegativity difference was sufficient, the shape plays a crucial role in determining the polarity of the molecule.

The polarity of carbon tetrachloride is a similar scenario.

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Figure 5: Carbon tetrachloride molecule

The electronegativity difference between carbon and chlorine is sufficient to polarize the C-Cl bond. The pair of electrons shared between C and Cl is directed more towards the Cl atoms. However, the tetrahedral carbon molecule has a symmetrical tetrahedral shape, which results in the net dipole moments of the bonds being broken, resulting in a net dipole moment of zero. Hence the molecule becomes non-polar.

Image courtesy:

  1. "Hydrogen-Fluorid-3D-vdW" ByBenjah-bmm27- Own work accepted (based on copyright claims) (Public Domain) via Commons Wikimedia
  2. "Ammonium-2D" by Lukáš Mižoch - Own work (Public Domain) via Commons Wikimedia
  3. “Carbon Dioxide” (Public Domain) via Commons Wikimedia
  4. “Carbon Tetrachloride 3D Spheres” (Public Domain) via Commons Wikimedia

Reference:

  1. "Why is the carbon tetrachloride molecule non-polar and yet the bonds in it are polar?" Sokratisch.org. Np, nd web. February 13, 2017.
  2. “Is ammonia polar?” Reference.com. Np, nd web. February 13, 2017.
  3. Ophardt, Charles E. “Molecular Polarity”. Virtual Chembook. Elmhurst College, 2003. Web. February 13, 2017.

About the author: Pabasara

Pabasara has a bachelor's degree in chemistry and reads for M.Phil. in chemistry. She has work experience in both academic and industrial settings.