[GeLe5000]

Hello everyone.

I'm told that Nitrogen, contrarily to Phosphorous, can't link to 5 Chlorine atoms to form NCl₅, whilst Phosphorus can form PCl₅ besides PCl₃ (only NCl₃ in the case of Nitrogen).
In order to Phosphorous to  link to 5 Chlorine atoms, there's a disappariement of 3s² to place 1 electron (promotion) in a d orbital, which gives 5 free electrons (3s¹, 3p³, 3d¹).

They explain that NCl₅ doesn't exist because the electronic structure of level L (n = 2), characteristic of atom N, behaves no d orbital.
In another book, they say that the promotion to an n + 1 orbital is never observed because it would require too much energy (activation).

These are two good explanations for the inexistence of NCl₅.

However, there comes a question.

When an electric current goes through a hydrogen gas (H2), Hydrogen atoms are separated and the electron of atom H, excited, goes to orbits n = 2, 3, 4, etc. (or orbital 2s, 3s, 4s, etc., I suppose), hence the well-known absorption spectrum.
If a Nitrogen gas is submitted to the same treatment, where will go the excited electrons?
Will they be sent to the infinite because there's no orbital available farther than orbitals p?

Thank you for your ideas.

[mjc123]

Will they be sent to the infinite because there's no orbital available farther than orbitals p?

Who says there isn't? Electrons will be excited to higher orbitals, and then fall back to the ground state, emitting radiation.
It's not that N has no higher orbitals; the issue is the size of the energy gap between them and the ground state. There are no 2d orbitals; the next lowest would be 3s, but the energy gap between 2p and 3s is much bigger than from 3s to 3d. If you had to put this much energy in, would you get it back from the energy of the bonds you could form?
Furthermore, the P atom is considerably larger than the N atom. You can comfortably fit 5 Cl atoms around P (even 6, in PCl₆^-), but I suspect you couldn't fit 5 Cl atoms around N at the equilibrium N-Cl bond distance. The repulsion between the Cl atoms would considerably weaken the bonds.
All of which suggests that if you could somehow make NCl₅, the decomposition NCl₅  NCl₃ + Cl₂ would be very favourable.

[GeLe5000]

It's perfectly clear now.
All the possible orbitals are always available, but when some begin to be missing at some level, it's necessary to go to the next level, which requires much energy. And NCl₅ would anyway be an unnatural molecule, because it's too unstable, also for steric reasons.

Thank you very much.

[Mitch]

@mjc123. Perfect answer.

[mjc123]

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