[boodyx1]

write the total Hamiltonian for li 3 atom
then mention what are the diff bet the same orbitals and their
 energies in lithium  and Hydrogen atom.
this an important question for me ....plz help me since it may be in my exam tomorrow

[tamim83]

The Li Hamiltonian should have kinetic energy terms for each electron and the nucleus and potential energy (Coulomb's law) terms for each interaction.  So each electron will interact with the nucleus and with one another.  Also, remember the charge on the Li nucleus.  This should help you get started.  It would help to look at the Hamiltonian for the H atom; the one for Li will just have more terms.  

For the orbitals, the difference will come mainly from the fact that Li has more electrons (if you are talking about the neutral atom).  In the H atom, all of the orbitals of the same n quantum number have the same energy.  This won't be the case in Li (or any atom with more than one electron) because of shielding.  So orbitals with the same n will not have the same energy (but those with the same l will).  The orbitals that can get closer to the nucleus will be lower in energy.  To figure out orbital energy ordering (same n), look at the radial distribution functions for the orbitals to see which ones have "hills" closest to r=0 (the nucleus).  

Hope this helps some