[kayvanfar67]

here is my question:

we know that (s) and (o2) can make a covalent bond.
now please tell me why (s) and (o) can not do this?

 

[movies]

What do you mean?  There are examples of S-O bonds which are covalent.  S can't bond to O₂ though because O₂ already has a full octet.

[kayvanfar67]

i mean that S and O2 can make a molecules with covalent bond. but S and O cannot make a molecules with covalent bond. so why?  
you are right my question was not right. i'm sorry for this.  

[Donaldson Tan]

Are you asking why sulphur reacts with the diatomic oxygen molecule to form covalent product, but not react with oxygen radical? If this is what you are asking, I can tell you is that forming the O=O bond is more thermodynamically favoured than S-O or S=O bonds because being small and the same size, the degree of overlap of the O valence orbitals would be greater, and thus stronger bond, and hence more exothermic than forming the sulphur-oxygen bonds. Hence, the oxygen radicals are more likely to react with each other before reacting with sulphur.