Rationalize the ff. in terms of stabilities of sigma and pi bonds: n2 is the common form of n & p4 for p?

Question

Also, explain why the most common form of Oxygen is O2 whereas the msot common form of sulfur is s6.

Answer 1

You mean S8 as the most common form of sulfur.

These are some of the more obvious results of a somewhat general trend -- the 2nd period elements like O, N, and to a lesser extent C are very good at forming pi-bonds. In fact, contrary to usual dogma, the pi bonds in N2 and O2 are actually stronger than single bonds for those elements (for reasons that are often explained using some handwavy waffle about lone pairs, but you really need MO theory to make proper sense of it). In contrast, elements like Si, P, and S make extremely poor pi-bonds to themselves, and only marginally decent pi-bonds even to oxygen.

Some other examples: CO2 is a molecular gas with C=O double bonds, SiO2 forms only single bonds in an extended network, forming quartz. Similarly, acetone is Me2C=O, silicone is a polymer of -[-O-SiMe2-]- single bond linkages. [NO3]– is a discrete molecular ion with delocalized N=O pi bonding, [PO4]3– tends to associate in chains of tetrahedra with extra single bonds and reduced pi-bonding (as seen in ATP, for example). SO3 exists in equilibrium with a trimeric form that has a ring of S-O-S-O-S-O- single bonds. NO is a stable diatomic compound, NS forms a weird associated tetrameric complex with a puckered ring of S-N-S-N- links.

Basically what you're looking at is thermodynamics of the bonds you're choosing between. Compare N2 associating to form either a hypothetical N6 ring (analogous to HCCH trimerizing to form benzene, which is downhill) or the N4 tetrahedra similar to what phosphorus does. To do that reaction, you need to break one (or two) pi bonds per N2, and form two new sigma bonds. If the two sigma bonds are stronger, it does that. If the pi-bond is stronger, it won't. Pi is a very strong bond for N, and N-N single bonds are unexpectedly weak, so that's a bad deal. N2 stays a diatomic molecule with a triple bond. pi bonds are terrible for P, the single bonds are OK, so P2 isn't stable, forming the tetrahedra instead. Similarly, pi-bond in O2 is great, single O-O are weak, so O2 is a discrete molecule; the reverse is true for S, so you get rings and chains of S atoms under normal conditions.