" n " is called principle quantum number. Here , n = 2 which means , number of shells = 2 .: l ( azimuthal quantum no: ) / ( n - 1 ) = 0 , 1 & m ( magnetic quantum number ) = -1 , 0 , +1 ..... s ( spin quantum number ) = ± 1/2 ..... Coming to this question , here n = 2 .... l = 1 ( possible ) ..... ml = -2 [ not possible , because it can only be a number from this set ( -1 , 0 , +1 ) ] ..... .: this set is not possible