The bonds in carbon dioxide have an electronegativity difference of 1.0 whereas the electronegativity difference in the oxygen molecule is 0.0. How would you explain to a grade 10 student that both of these molecules are gases at room temperature?

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Explanation:

All gases are held together by weak dispersion forces. One would now ask; Since the electronegativity difference between carbon and oxygen is about 1.0 while the electronegativity difference between two oxygen atoms is 0.0, why are they both gases at room temperature?

The answer lies in the recognition of two facts;

1) The dipole moment is a vector quantity so we also consider its direction

2) possession of polar bonds does not necessarily make a polar molecule.

We all know that a polar molecule will have greater intermolecular forces between its molecules and will most likely be a liquid instead of a gas. However,if we look at the overall dipole moment in CO2, we will notice that the direction of the dipole moment due to the two C-O bonds are opposite to each other hence they cancel out each other leaving the CO2 molecule with a zero effective dipole moment just as oxygen gas.

Hence, possession of polar bonds does not necessarily make a polar molecule. Therefore both oxygen and CO2 are gases at room temperature.

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