[jennielynn_1980]

hi all.

The question I have is why is C₂₀H₄₀ solid at 25 C but C₄H₈ is a gas?
I know that the basic reason is that because C₂₀H₄₀ has more atoms there are greater forces of attraction within the molecule making it solid at this temp.  HOwever, I can't figure out WHICH forces I am considering.  I think it is van der Waals forces but I am not totally sure about this because I am not totally sure if I am dealing with non-polar molecules.  The other hydrocarbons I have looked at  have carbon atoms surrounded completely by hydrogen atoms.  I this case I am guessing the arrangement would leave two carbons on either end that are NOT bonded to hydrogens.  Would this make it polar or non-polar? 

Thanks

[tamim83]

Hi
C20H40 either has a double bond or a ring in it someplace (there are dozens of isomers making dozens of possibilites.  Same with C4H8, only it has less isomers.  I think it is just VDW forces here (although there may be polar isomers of C4H8  ).  My explanation would be  more atoms, more VDW forces, and the more tightly packed the compund is. 

[jennielynn_1980]

Okay, thanks.  That was my original thought because we haven't studied hydrocarbons yet.  The chapter I just did was on periodicity and chemical bonding so I don't think I need a complex answer.

[skyglow1]

The C-H bond is not very polar, so in anywhere in either molecules there is not much polarity. Van Der Waals are the only forces you are considering