dπ-pπ bond requires a d orbital. Nitrogen (Period 2, Z=7) has configuration 1s²2s²2p³ — it has NO d orbitals in its valence shell.
Oxygen (Period 2, Z=8) also has no d orbitals.
For dπ-pπ bonding: one atom needs a d orbital (donor) and the other a p orbital. Since BOTH N and O are Period 2 — neither has d orbitals → dπ-pπ bonding is impossible between them.
❌ WRONG: "N can form dπ-pπ bond with O"
✅ CORRECT: N forms only pπ-pπ bonds (N=N in N₂, N=O in NO₂). Both N and O are Period 2 — no d orbitals available.
✅ Option 1 CORRECT: P and As are Period 3+ → have d orbitals → form dπ-dπ bonds with transition metals (back-bonding in metal complexes)
✅ Option 3 CORRECT: N₂ has N≡N (pπ-pπ triple bond), N=N in hydrazine derivatives
✅ Option 4 CORRECT: P (P₄), As, Sb show catenation — form M-M bonds
pπ-pπ bonding: both atoms use p orbitals to form a pi bond. Only possible for small atoms where p-p orbital overlap is good — Period 2 elements (C, N, O). Examples: N≡N (in N₂), C=C (alkenes), N=O (in NO, NO₂). dπ-pπ bonding: one atom uses a d orbital and the other uses a p orbital. Requires at least one Period 3+ atom (which has d orbitals). Examples: S=O in SO₂ (S uses 3d, O uses 2p), P=O in phosphoryl compounds. dπ-dπ bonding: both atoms use d orbitals — seen in metal complexes where ligands like P(C₂H₅)₃ donate back electrons via d orbitals.
📌 Nitrogen (Period 2): 2s²2p³ — only s and p orbitals → max bond order through pπ-pπ only → N₂ has triple bond (N≡N, bond energy = 946 kJ/mol — strongest diatomic)
📌 Phosphorus (Period 3): 3s²3p³3d⁰ — has empty 3d orbitals → can form dπ-pπ bonds → P=O bonds in H₃PO₄ → also expands octet (PCl₅, PF₅)
📌 Key Rule: Period 2 elements (C,N,O,F) — NO dπ bonding. Period 3+ (P,S,Cl,Si...) — CAN form dπ bonding
📌 This explains why N₂O₅ is N-O-N (no N=O with d orbital), but P₂O₅ has P=O bonds
N₂ has a triple bond (one σ + two π, all pπ-pπ). Bond energy = 946 kJ/mol — highest of any diatomic molecule. This extreme stability makes N₂ chemically inert at room temperature — despite nitrogen being 78% of atmosphere, plants cannot use it directly (nitrogen fixation is needed). The two π bonds in N₂ are both pπ-pπ type (N is Period 2, uses 2p orbitals). For comparison: P₄ has only single P-P bonds (P uses 3p orbitals; pπ-pπ overlap with 3p is poor due to larger size and diffuse orbitals).
Catenation = forming bonds with like atoms. In Group 15: N shows very limited catenation (N-N bond in hydrazine N₂H₄ and a few others) because N-N single bond is relatively weak (163 kJ/mol) due to lone pair-lone pair repulsion between adjacent N atoms. P, As, Sb show much better catenation: P₄ (tetrahedral P₄ molecules), As₄, Sb₄, polyphosphides (P₄²⁻, P₅⁻ chains). The larger atoms have orbitals that overlap better for homonuclear bonding with less lone pair repulsion. Bi shows least catenation in Group 15.
📌 N oxoacids: HNO₂ (nitrous), HNO₃ (nitric) — N uses only pπ-pπ bonding with O
📌 P oxoacids: H₃PO₂, H₃PO₃, H₃PO₄, H₄P₂O₇ — P uses dπ-pπ bonding (P=O)
📌 Basicity of P acids: number of P-OH groups = number of replaceable H (basicity). P=O and P-H are NOT acidic.
📌 H₃PO₂ (hypophosphorous): 1 P-OH → monobasic. H₃PO₃ (phosphorous): 2 P-OH → dibasic. H₃PO₄ (phosphoric): 3 P-OH → tribasic
📌 Both H₃PO₂ and H₃PO₃ are good reducing agents (they have P-H bonds)
White phosphorus (P₄): tetrahedral structure, 60° P-P-P angle (strained), highly reactive and toxic, glows in dark (chemiluminescence), stored under water. Red phosphorus: polymeric, less reactive, non-toxic, used in matchboxes. Black phosphorus: most stable, graphite-like layered structure, semiconductor. Violet phosphorus: monoclinic, most thermodynamically stable. Conversion: white → red (heating, 250°C); white → black (high pressure). White phosphorus dissolves in CS₂ (carbon disulphide); red phosphorus does not — used to distinguish them.
📌 N₂O (nitrous oxide, +1): laughing gas, anaesthetic, linear
📌 NO (nitric oxide, +2): colourless, paramagnetic (odd e⁻), turns brown in air
📌 N₂O₃ (dinitrogen trioxide, +3): blue solid, anhydride of HNO₂
📌 NO₂ (nitrogen dioxide, +4): brown gas, paramagnetic, acidic, smog component
📌 N₂O₄ (dinitrogen tetroxide, +4): colourless, NO₂ dimer
📌 N₂O₅ (dinitrogen pentoxide, +5): white solid, anhydride of HNO₃
Atmospheric N₂ must be "fixed" (converted to useful compounds) before plants can use it. Natural fixation: lightning converts N₂ + O₂ → NO → HNO₃ (about 5-8%); Biological fixation by Rhizobium bacteria (in legume root nodules) using nitrogenase enzyme — converts N₂ → NH₃. Industrial fixation: Haber process — N₂ + 3H₂ ⇌ 2NH₃ (Fe catalyst, 400-500°C, 200 atm, high pressure). Ostwald process converts NH₃ → HNO₃. About 50% of world food production depends on synthetic nitrogen fertilisers made this way.