Can someone explain the answer to this question to me?
Arrange the following in order of increasing Melting point, with the lowest first: H2O, NaCl, CH4, MgO.

the answer is Ch4

Question

Can someone explain the answer to this question to me?
Arrange the following in order of increasing Melting point, with the lowest first: H2O, NaCl, CH4, MgO.

the answer is Ch4<H2O<NaCl<MgO

Can someone explain why this is the answer and how you would know that?

aaronq · Answer

melting point is due to intermolecular forces which hold molecules together.. there is a hierarchy of strength in IMF's .. london dispersion forces<dipoles (there are several types ion-induced dipole, dipole-dipole, H-bonding, etc)<ionic

so classify your molecules in terms of these forces.. 

CH4 only has london dispersion so its last
H2O is capable of H-bonding (N,F and O when bonded directly to H can H-bond)
NaCl is ionic
MgO is also ionic but the charges are +2 and -2 so they are greater than NaCl which are only +1 and -1, so they're held tighter

anonymous · Answer

ohh ok, I guess I need to brush up on my intermolecular forces. Thank you! :)

aaronq · Answer

no prob !