how to know which compounds acquire octet state?why copper compounds does not acquire octet state?

Dear Shyam,

Elements in the 3rd and higher periods may have more than 8 valence electrons. A possible explanation for this is to say that these atoms have d-type atomic orbitals to accommodate more than 8 electrons. In the following molecules, the number of valence electrons in the central atoms are as indicated:

   OH
   |
 O=S=O  
   |
   OH

Draw the Lewis dot structures for the above molecules, and count the number of valence electrons for the central atoms. For H₂SO₄, the S atom has 12 electrons in the structure shown on your right. Each dash represent a chemical bond, which has two electrons. There is a total of 6 bonds around the S atom, and therefore 12 electrons.

When B, Be, and some metals are the central atoms, they have less than 8 valence electrons. The following compounds do form, but the octet rule is not satisfied. These are electron defficient molecules. 

Another case of exception to the octet rule are molecules with odd number of valence electrons. For example: