Final answer:
NCl₅ cannot exist because nitrogen's valence shell cannot accommodate more than eight electrons, adhering to the octet rule. PCl₅ can exist because phosphorus can use d orbitals to expand its valence shell and accommodate more than eight electrons, allowing it to form five covalent bonds.
Step-by-step explanation:
The reason why NCl₅ cannot exist but PCl₅ can is primarily due to the differences in the electron configurations and the valence shell capacities of nitrogen and phosphorus. Nitrogen, with the electronic configuration of 1s² 2s² 2p³, does not have d orbitals available in its valence shell, meaning it can accommodate only a maximum of eight electrons, adhering to the octet rule. Thus, nitrogen can form a maximum of four covalent bonds, one of which is a triple bond, as in nitrogen gas (N₂), or three single bonds, as in ammonia (NH₃).
Phosphorus, on the other hand, has the electronic configuration of [Ne] 3s² 3p³ and can expand its valence shell to include the 3d orbitals because it is in the third period of the periodic table. This allows phosphorus to accommodate more than eight electrons and form five covalent bonds, as found in PCl₅. The molecular structures of PCl₃ and PCl₅ demonstrate phosphorus's ability to form three and five bonds respectively, which is due to this capacity to have an extended valence shell. PCl₅, when it reacts with water, forms phosphoric acid, H₃PO₄, showing its stability as a compound, unlike a hypothetical NCl₅, which cannot form due to the limitations posed by nitrogen's valence shell.