[Tomassci (OP)]

We all know as an important acid in manufacture and agriculture. However, when it comes to phosphoric acid, just straight-up refuses to exist, even though phosphorus is right under nitrogen in the periodic table.

Why does phosphorus behave in its own way? Why can't it be normal?

[alancalverd]

https://pubchem.ncbi.nlm.nih.gov/compound/Metaphosphoric-acid

tells you how to make it and what it is used for. Interestingly it causes skin burns and eye damage, and its principal use is in cosmetics!

[Bored chemist]

Arsenic and antimonic acids are typically of the type H3XO4 (Though the meta versions exist).
So the question is why doesn't nitrogen usually follow the rules?
And the answer is largely that it's small.
Though Na3NO4 etc do exist
https://en.wikipedia.org/wiki/Orthonitrate
So it seems they all follow both sets of rules.

[chiralSPO]

One can think of it as a hydration/dehydration equilibrium:
H₂O + HXO₃ H₃XO₄

There are two reasons that the equilibrium favors the left-hand side when X = N, but the right-hand side when X = P, As, (or Sb, I believe):

1) (the simple answer, pointed out by Bored chemist) nitrogen is the smallest of the bunch (covalent radius of N = 75 pm, P = 106 pm, and As = 119 pm), so it is simply harder to fit the extra oxygen on there.

2) (the complex answer) the 2p orbitals on nitrogen and oxygen have fairly similar energies** (–13.1 and –15.9 eV vs vacuum, respectively) as well as very similar sizes (as noted in point 1), as well as shorter σ bonds (also due to point 1). This means that the p orbitals on neighboring O and N atoms can form very strong π interactions, and that the electrons in the π bonding and π anti-bonding orbitals would both be shared fairly equally.

In contrast, the valence of the phosphorus atom is a 3p orbital, with a much higher energy** (–10.2 eV; again comparing to –15.9 eV for O), and extends much farther away from the nucleus (but not towards a neighboring atom).

Screen Shot 2021-11-29 at 4.15.13 PM.png (42.56 kB . 794x624 - viewed 2458 times)

This means that in HNO₃ the nitrogen atom has much more electron density compared to the P atom in HPO₃ (the N is less electrophilic than the P, making the N less likely to accept electron density from another O), while the oxygen atoms bound to N have less electron density than those bound to P (the P-bound O atoms are more basic than N-bound, making them more likely to gain H⁺) and much stronger N=O bonds than P=O bonds (so it is easier to break the double bonds).


Screen Shot 2021-11-29 at 8.08.24 PM.png (45.63 kB . 546x680 - viewed 2421 times)

* https://www.schoolmykids.com/learn/interactive-periodic-table/covalent-radius-of-all-the-elements
** https://www.colby.edu/chemistry/PChem/notes/AOIE.pdf