Question

How do you determine if a molecule has a polar covalent bond based on the lewis dot structure?

Answer 1

When you draw the lewis dot structure can you draw a circle around both atoms that represent a stable inert gas structure.

Explanation:

In a covalent bond the electrons are shared between both atoms, in a roughly equal manner. If the electron structures for both atoms are stable after sharing they will share the electrons covalently.

`CO_2` as an example

.. ..
: O :: C :: O :

If you draw a circle around either Oxygen it will have 8 electrons a stable structure. The four electrons from the double bond and the four electrons from the two pairs of unshared electrons on the Oxygen.

If you draw a circle around the Carbon it will have 8 electrons a stable structure. The two sets of four electrons from the double bonds joining the Carbon to the two Oxygens.

`NaCl` as an example
..
Na: Cl:
..

If you draw a circle around the Sodium it has only 2 electrons which is not a stable structure. The Sodium will lose the 2 electrons that form the bond with Chlorine. The losing rather than sharing the electrons creates an ionic bond.

Answer 2

You calculate the electronegativity differences (`ΔEN`) between the central atom and the atoms that are directly attached to it.

Explanation:

For example, the Lewis structure of `"CO"_2` is

Look up the electronegativities of `"C"` (2.55) and `"O"` (3.44).

`ΔEN = "3.44 - 2.55" = 0.89`

This value is greater than 0.5, so the `"C=O"` bond is polar covalent.

Now consider the Lewis structure for `"PH"_3`:

The electronegativities are `"P" = 2.19` and `"H" = 2.20`.

`ΔEN = "2.20 - 2.19" = 0.01`

This is about as nonpolar as you can get for a bond between two different atoms.

Conclusion: `"PH"_3` has no polar bonds.