The following sets of quantum numbers ( n, I, ml, ms ) that cannot occur within an atom of gallium are :
-  4, 2, 1, ½ ( B ) 
-  4, 1, 2, ½ ( D ) 
-  4, 3,1, 1/2 ( E )
Determine the sets of quantum numbers that cannot occur within an atom of gallium 
Given that
Ga ( 31 ) = 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p¹
                  [ 4r ] 3d¹⁰ 4s²4p¹
where : n = principal quantum numbers,  l = azimuthal quantum numbers, ml = magnetic quantum numbers, ms = spin quantum numbers.
For option B 
Given that 4d shell is absent for Gallium the set cannot occur within the atom of Gallium 
For Option D and E 
All quantum number cannot occur here because the value of l = 2 for option D while for option E the value of l = 3 
Hence we can conclude that sets of quantum numbers ( n, I, ml, ms ) that cannot occur within an atom of gallium are as listed above. 
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