Polar Molecules

Polar molecules

Some molecules with polar covalent bonds are polar molecules,e.g. \(\text{H}_{2}\text{O}\). But not all molecules with polar covalent bonds are polar. An example is \(\text{CO}_{2}\). Although \(\text{CO}_{2}\) has two polar covalent bonds (between\(\text{C}^{\delta^{+}}\) atom and the two \(\text{O}^{\delta^{-}}\) atoms), the molecule itself is not polar. The reason is that \(\text{CO}_{2}\) is a linear molecule, with both terminal atoms the same, and is therefore symmetrical. So there is no difference in charge between the two ends of the molecule.

Definition: Polar molecules

A polar molecule is one that has one end with a slightly positive charge, and one end with a slightly negative charge. Examples include water, ammonia and hydrogen chloride.

Definition: Non-polar molecules

A non-polar molecule is one where the charge is equally spread across the molecule or a symmetrical molecule with polar bonds. Examples include carbon dioxide and oxygen.

Tip:

To determine if a molecule is symmetrical look first at the atoms around the central atom. If they are different then the molecule is not symmetrical. If they are the same then the molecule may be symmetrical and we need to look at the shape of the molecule.

We can easily predict which molecules are likely to be polar and which are likely to be non-polar by looking at the molecular shape. The following activity will help you determine this and will help you understand more about symmetry.

Optional Activity: Polar and non-polar molecules

The following table lists the molecular shapes. Build the molecule given for each case using jellytots (chewing gums or any other similar substance that’s easy to find) and toothpicks. Determine if the shape is symmetrical. (Does it look the same whichever way you look at it?) Now decide if the molecule is polar or non-polar.

Geometry

Molecule

Symmetrical

Polar or non-polar

Linear

\(\text{HCl}\)

  

Linear

\(\text{CO}_{2}\)  

Linear

\(\text{HCN}\)  

Bent or angular

\(\text{H}_{2}\text{O}\)

  

Trigonal planar

\(\text{BF}_{3}\)

  

Trigonal planar

\(\text{BF}_{2}\text{Cl}\)

  

Trigonal pyramidal

\(\text{NH}_{3}\)

  

Tetrahedral

\(\text{CH}_{4}\)

  

Tetrahedral

\(\text{CH}_{3}\text{Cl}\)

  

Trigonal bipyramidal

\(\text{PCl}_{5}\)

  

Trigonal bipyramidal

\(\text{PCl}_{4}\text{F}\)

  

Octahedral

\(\text{SF}_{6}\)

  

Octahedral

\(\text{SF}_{5}\text{Cl}\)

  

Example: Polar and non-polar molecules

Question

State whether hydrogen (\(\text{H}_{2}\)) is polar or non-polar.

Step 1: Determine the shape of the molecule

The molecule is linear. There is one bonding pair of electrons and no lone pairs.

Step 2: Write down the electronegativities of each atom

Hydrogen: \(\text{2.1}\)

Step 3: Determine the electronegativity difference for each bond

There is only one bond and the difference is \(\text{0}\).

Step 4: Determine the polarity of each bond

The bond is non-polar.

Step 5: Determine the polarity of the molecule

The molecule is non-polar.

Example: Polar and non-polar molecules

Question

State whether methane (\(\text{CH}_{4}\)) is polar or non-polar.

Step 1: Determine the shape of each molecule

The molecule is tetrahedral. There are four bonding pairs of electrons and no lone pairs.

Step 2: Determine the electronegativity difference for each bond

There are four bonds. Since each bond is between carbon and hydrogen, we only need to calculate one electronegativity difference. This is: \(\text{2.5} – \text{2.1} = \text{0.4}\)

Step 3: Determine the polarity of each bond

Each bond is polar.

Step 4: Determine the polarity of the molecule

The molecule is symmetrical and so is non-polar.

Example: Polar and non-polar molecules

Question

State whether hydrogen cyanide (\(\text{HCN}\)) is polar or non-polar.

Step 1: Determine the shape of the molecule

The molecule is linear. There are four bonding pairs, three of which form a triple bond and so are counted as \(\text{1}\). There is one lone pair on the nitrogen atom.

Step 2: Determine the electronegativity difference and polarity for each bond

There are two bonds. One between hydrogen and carbon and the other between carbon and nitrogen. The electronegativity difference between carbon and hydrogen is \(\text{0.4}\) and the electronegativity difference between carbon and nitrogen is \(\text{0.5}\). Both of the bonds are polar.

Step 3: Determine the polarity of the molecule

The molecule is not symmetrical and so is polar.

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This is a lesson from the tutorial, Bonding and Atomic Combinations and you are encouraged to log in or register, so that you can track your progress.

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