# Why does sulphur exist in two forms in solid state, monoclinic and rhombic? Why in one favoured over another at a certain temperature and pressure?

Both Rhombic and Monoclinic sulphur consist of $$S_8$$ molecules packed together. Yet the former is more stable below $$114^{\circ}C$$ at 1 atm and spontaneously converts to monoclinic sulphur above that temperature. Does this have something to do with the crystal structure? Gibbs equation tells us that structures with higher entropy are preferred at a higher temperature.

Even at the same temperature we can see that rhombic form is more common at higher pressures. Is pressure decreasing the entropy and favouring one form over another?

Does that mean a monoclinic structure has higher entropy and lower enthalpy, if that's true why is entropy higher? Or is my hypothesis wrong and it's based on something completely different.

• "Does this have something to do with the crystal structure?" Yes. ;-)
– Karl
Aug 29, 2021 at 12:00
• @Karl, and could you give me a hint please : )(or even the answer if you want to) Aug 29, 2021 at 12:22
• Well, have you had a look a the two crystal stuctures of sulphur? (I haven't, that's your homework. ;) )
– Karl
Aug 29, 2021 at 12:33
• @Karl I suppose the monoclinic structure will have lesser symmetry, but if this answer is to be believed it's not that straightforward. physics.stackexchange.com/questions/43492/… Now I'm in high school and the only book for physical chem I know about is Atkins, maybe you can suggest some other one : ) Aug 29, 2021 at 13:00
• @NilayGhosh, I'm trying not to be too attached to internet points but it's difficult I guess lol Sep 1, 2021 at 4:37