June 02, 2020, 10:43:35 AM
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Topic: Why does PO3- ion undergo polymerization easily whereas NO3- does not?  (Read 6431 times)

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Offline adianadiadi

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Why does PO3- ion undergo polymerization easily whereas NO3- does not?

I think the reason might be: stronger N=O bond. But P=O bond is also stronger. So any idea? ???

Offline rabolisk

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Probably because nitrogen, as a period 2 element, is incapable of forming more than 4 bonds, whereas phosphorus is.

Offline adianadiadi

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There is no question of forming 4 bonds during oligomerization.

Offline jcjlf

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There is no question of forming 4 bonds during oligomerization.
Indeed.
In the Lewis-structure of NO3- you have an electronic structure with one N=O bond, relatively stable because in period 2 the pi-pi orbitals do overlap pretty good, for the atoms are small.
In period 3 the atoms have a greater size, so these elements don't form strong double bonds, the P=O in a similar structure as in NO3- is weak or hardly existing because pi-pi interaction is weak. Thus PO3- is an unstable species on its own.
So PO3- easily adds up to PO43- to form P2O74-, a more stable dimer with only single P-O bonds. And this can go on with the next PO3- attaching to form a trimer P3O105- etc. etc.

This explanation disregards p-d orbital interaction in phosphates, but it is of some help. There is a general tendency to form only single bonds in the third (and higher) period, e.g. compare CO2 and SiO2.

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