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you should look at the bonds i guess
Okay - the more the inter-molecular forces between the molecules, the easier to compress them, and hence liquefy them. -.-
So, in here, I guess you need to check the polarity of the bonds - More the electronegativity difference between participating atoms, more the dipole charge, and hence... more the inter-molecular attractions!
F-F ---> Same atom, same electronegativity, so just an instantaneous dipole. Not so great.
H\(_2\) -----> Same as above -_-
Ar -----> Noble gas BAH! very very veryyyyyyy little interactions with 'neighboring' molecules (selfish atoms -.- )
SiH\(_4\) ---> Interesting? Okay yes, a lot more interactions, and we have a dipole as well.
However, while interacting with the surrounding molecules, the Si atoms are always enclosed within the 'tetrahedral-sphere' of their molecules, and it's just H-atoms which (being on the periphery) interact with the outside world. And now, since all the H-atoms have the same type of charge, it leads to a net repulsive-atmosphere between the atoms, rather causing inter-molecular interactions! So, liquefaction in comparatively tougher for this one.
Which leaves us with H-F - (ofcourse this is the answer - what were you thinking eh? no surprises and no points for guessing -.-)
Why - because they form a linear molecule, with a good dipole (OMG H and F). So:|dw:1343201393276:dw|