Account for the following observations: (a) Phosphorus forms a pentachloride, but nitrogen does not. (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) is a monoprotic acid. (c) Phosphonium salts, such as \(\mathrm{PH}_{4} \mathrm{Cl}\), can be formed under anhydrous conditions, but they cannot be made in aqueous solution. (d) White phosphorus is more reactive than red phosphorus.

(a) Phosphorus can form a pentachloride (\(\mathrm{PCl}_{5}\)) due to its ability to form five covalent bonds, while nitrogen has only three bonding orbitals and can form a maximum of three covalent bonds (e.g. \(\mathrm{NCl}_{3}\)). (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) is a monoprotic acid because only one O-H bond in its structure can ionize and release a proton, while the other two P-H bonds do not ionize in solution. (c) Phosphonium salts, such as \(\mathrm{PH}_{4}\mathrm{Cl}\), can form under anhydrous conditions due to the reaction between phosphine and a halogen, but in aqueous conditions, phosphine reacts with water to form phosphoric acid and hydrogen gas, preventing the formation of phosphonium salts. (d) White phosphorus is more reactive than red phosphorus due to its weaker and strained P-P bonds in its P4 tetrahedron-shaped molecules, while red phosphorus has a more stable polymeric structure with stronger P-P single bonds.