Answer: Phosphorous
Explanation: Nitrogen(N): 7: \(1s^22s^22p^3\)
Phosphorus(P): 15:\(1s^22s^22p^63s^23p^3\)
Oxygen(O): 8:\(1s^22s^22p^4\)
Sulphur(S): 16:\(1s^22s^22p^63s^23p^4\)
As nitrogen contains half filled configuration, it is stable and does not react. Phosphorous lies below nitrogen and reactivity decreases down a group for non metals as they tend to gain electrons as the valence shell move away from the nucleus.
Oxygen has 4 valence electrons, thus it need to gain two electrons to attain stable configuration. Thus is reactive.
Sulphur lies below oxygen and also need to gain two electrons. Reactivity decreases down a group for non metals as they tend to gain electrons as the valence shell move away from the nucleus.
