Elements and Compounds

How does the size of a nitrogen molecule compare to that of oxygen?

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09/12/2011

Molecular size a bit tricky. As a quick comparison, we can use the covalent radius defined as 1/2 the distance between to identical covalently bonding nuclei. This is measured in picometers (1 pm= 1x 10-12 m). Nitrogen's covalent radius is 75pm so the length of a nitrogen (N2) molecule ought to be 4 X 75pm or 300 pm. A molecule of oxygen (O2) ought to be just a shade smaller 4 X 73pm or 292pm. So an oxygen molecule is a little less than 3% smaller than a nitrogen molecule.

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This isn't the best response, as it does not take into account the different bonding which is present in the two molecules.

Although one could use covalent radius, it is more appropriate to use a covalent radius determined from similar bonding.

For nitrogen, the covalent radius for a triple bond is 54 pm.

For oxygen, the covalent radius for a double bond is 57 pm.

(Numbers from Wikipedia...)

From these numbers alone, one would expect diatomic nitrogen (with its triple bond) to be smaller than diatomic oxygen (with its double bond).

These numbers are similar to bond lengths for diatomic nitrogen and diatomic oxygen of 110 pm (~2 x 54 pm) and 121 pm (~2 x 57 pm), respectively.