  
 Exa-8.6   
 
 The Oxygen atom has a configuration of 1s2,2s2,2p4. 4 electrons in the outer most shell.   
 
 Let us maximize the net spin  by assigning a spin of 1/2 to 3 of them but the fourth should have spin of -1/3. Hence S=3/2-1/2=1.   
 
 The consistent values of L for the 3 electrons are 1 -1 and 0.To maximize Ml, assign a L 0f +1 to the fourth electron.Thus L=1 & S=1 are the ground state quantum  
      numbers for Oxygen.     