Re: Why oxygen has low boiling point?

 Hello, Awais…

     Oxygen, and other small molecules, have low boiling points because their effective molecular 
weights are low. Note that weasely term "effective;" what's that doing in there?
      Boiling is the freeing of molecules (or atoms, for noble gases) from the surface of a liquid.  The 
escapees get the required kinetic energy from collisions with the other atoms/molecules in the liquid, 
and must also overcome the (usually weak) type of bonding that holds liquids together.  Given the same 
amount of energy from a collision (the probability of which is goverened by the temperature), a 
lightweight molecule is more likely to be knocked free of the surface than a heavier one.  Or, put 
another way, lighter molecules are knocked free at a lower temperature. The correlation isn't perfect, 
because the internal bonding in the liquids has some effect, but it is largely true.  
     The big exception is those molecules (water being the great example, because of hydrogen bonding) 
that have enough bonding, molecule to molecule, to have a higher effective molecular weight because 
the molecules stick together in "globs."  The globs are obviously heavier, and the higher molecule-
molecule bonding also makes it harder to break a glob free from the liquid surface.
     Consider, for another example, the straight-chain hydrocarbon series from ethane to decane.  
All of them are in many ways chemically similar, but the smaller and lighter ones boil at far lower 
temperatures:    
                              Molecule    Molec. Wt.   Boiling Pt, degr. C
                               ethane           30                -89
                               propane         44                -42
                               butane           58                 -1
                               pentane         72               +36
                               hexane          86               +68
                               heptane        100              +98
                               octane          114             +125
                               nonane         128             +151
                               decane          142             +174

   Water boils at 100C, meaning that its effective molecular weight is a tiny bit over 100, by comparing 
with heptane, or about the weight of 5 to 6 water molecules.  That is in line with Cotton and Wilkerson 
("Advanced Inorganic Chemistry", 4th ed, p. 98), who note that:

       "The structure of liquid water is not random as in liquids with more or less spherical non-polar 
molecules.  It is highly structured but with fortuitous H-bonded links and species of four- to seven-
membered rings, all of which are in rapid motion with the H-bonds bending, stretching, and 
occasionally breaking."  They list Symons, "Acc. Chem. Res." 1981, 14, 179 as a reference for this 
statement.  
        Note that 5 to 6 water molecules fits right into the middle of the range of  4-to-7 member rings.