[mrreluctant]

I have some problems in p-block elements.
1) Why does NH₃ form hydrogen bond but PH₃  does not ?
2) :delta:_eg for  O O^- and O O^2- are -141 Kj mol^-1
 and 702 mol^-1 respectively, which species O^- or O^2- will form large number of oxides and why?
3) Why is dioxygen is gas but sulphur a solid?

[bidiboom]

1) Why does NH3 form hydrogen bond but PH3  does not ?

Because we name the bonding of Hydrogen only with Nitrogen, Oxygen, or Fluorine, as "Hydrogen Bonding". Its reason, mainly, is that those 3 elements, N, O and F are the highest electronegative ones with little atomic size.. so they easily strip the electron of hydrogen and leave the hydrogen with a high positivity.. this positive pole attracts the negative pole of another nearby N, O or F strongly..

P is not that strong in pulling the electron of hydrogen, which means "not that electronegative", so cannot leave the hydrogen with a high positivity.. and so hydrogen cannot pull another nearby negative pole that strongly..

[bidiboom]

I had to cut my reply in two for there is a problem with the editor..

3) Why is dioxygen is gas but sulphur a solid?

Because they have different intermolecular forces. Lets examine one by one:
a) Hydrogen bonding: They dont have a H-N, H-O or H-F bond, so they dont have hydrogen bonds.
b) London dispersion forces: Dioxygen is little in size, but sulfur is stable in the form of S8, which is greater in size.. as the molecule gets bigger in its shape and it gives more surface, its electron interactivity increases as well.. also as the atom get bigger in size, its electron cloud gets far to the nucleaus and makes it more available for electrons drift temporarily.. actually if you read "polarizability" you can get a more clear info about this point.. so because S8 is greater in both atomic size and molecular size, its London dispersion forces is stronger than that of dioxigen.
c) Dipole-dipole forces: Both are non-polar, so they dont have dipole-dipole force.

So: Because the dispersion forces are stronger in S8 than in O2, the boiling point of S8 is much higher than O2.. and because O2 is boiling at -183C (which means it turns to gas state at a much lower temperature than room temperature, 25C), we see it as gas, and because S8 boils at 444C, we see it as a soft solid.   

[vmelkon]

So: Because the dispersion forces are stronger in S8 than in O2, the boiling point of S8 is much higher than O2.. and because O2 is boiling at -183C (which means it turns to gas state at a much lower temperature than room temperature, 25C), we see it as gas, and because S8 boils at 444C, we see it as a soft solid.   

Sulfur melting point is important. 112 C. Therefore it is a solid

[bidiboom]

So: Because the dispersion forces are stronger in S8 than in O2, the boiling point of S8 is much higher than O2.. and because O2 is boiling at -183C (which means it turns to gas state at a much lower temperature than room temperature, 25C), we see it as gas, and because S8 boils at 444C, we see it as a soft solid.   

Sulfur melting point is important. 112 C. Therefore it is a solid

Oh yes, you're right.. good point