What is the reason/s that P exhibits two difference valences 3 and 5?
Answer:
Phosphorus has 5 electrons in its outer shell so by sharing three of these with three electrons from other atoms it achieves a 'full' outer shell of eight electrons - hence valency three. However, (unlike nitrogen) phosphorus also has an empty 3d-subshell at a relatively low energy which is able to accept electrons. Thus, phosphorus can share all five of its electrons by first absorbing energy to promote one of them to the 3d-subshell and then forming five bonds (with sp³d hybridisation) with the consequent release of more energy - hence valency five
Two s-electrons, 3 p-electrons. Orbitals are most stable when empty, filled, and half-filled (d-orbitals).
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