Question

How many electrons in an atom can have the following quantum numbers

a)n=5 l=2 ms =+1/2

b)n=4 ms =-1/2

Answer 1

(a) n=5 and l=2 indicate that it is a 5d subshell,
      which has a maximum of 10 electrons out of which 5 electrons have +1/2          spin and other 5 have -1/2.
      So, the answer is '5'

(b) n=4 indicates that it is either 4s, 4p, 4d or 4f subshells having 2, 6, 10, 14       electrons respectively.
      Half of the electrons have +1/2 and the other half have -1/2 spins.

Therefore the total number of electrons having n=4 and spin=-1/2 are
   1+3+5+7=16 
                                   
I Hope it helps