Polarity

4.2.8 - Predict whether or not a molecule is polar from its molecular shape and bond polarities

Electronegativity

The ability of an atom in a molecule to attract electrons to itself

You can measure how polar a molecule is based on the difference in the electronegativities of the

constituent elements.

Based on its relative position on the periodic table, you can estimate the electronegativity of an

element. The arrows below show increasing electronegativity.

For example, if we look at OH:

We can see that oxygen has the higher electronegativity. Although it is one period lower, it is 15

groups to the right, which means that it is higher. So, as a molecule, OH would look like this:

OH is therefore a polar molecule. Since electrons actually exist in a probability cloud, it would

appear more like this:

When looking at molecules with more atoms, it is possible for the dipoles to be cancelled out

In the majority of cases, any molecule that is completely symmetrical will have cancelled dipoles.

This means that CH4, and many others, are non-polar molecules, even though the constituent

elements have different electronegativities.

Remember that the diatomic molecules (C2, H2, I2, N2, F2, O2, Br2) are also non-polar, since they are

symmetrical. This explains why they exist as gases.

Be careful to make sure that a molecule is really symmetrical. HCN, for example, is linear, but is polar

because it has different elements, with different electronegativities, on either side. CO2, on the other

hand, is non-polar, because the same element is found on each side.

Ionic bonds form when the electronegativity of one atom is so strong that it completely removes the

electron(s) from the other one altogether.

Each element has an assigned numerical value for their electronegativity. Therefore, we can

calculate the difference between the electronegativities of two elements, and hence its degree of

polarity.

The types of bonds are classified into the difference in electronegativity.

Difference in Electronegativity Bond Type

0.0 – 0.4 Non-polar covalent 0.5 – 2.0 Polar covalent > 2.0 Ionic

For example, the difference between C and O is 1. Therefore, a CO molecule would be polar

covalent. Be careful when calculating this to ensure that the dipoles do not cancel.

As we can see, the highest possible difference occurs between Cs and F – I would love to see that

reaction!!