Atomic number of Calcium #"Ca"# is 20.
We know that there are four quantum numbers which can be assigned to "describe" any electron in an atom. Each electron has a unique set of these four quantum numbers.
#n =# principal quantum number, which defines the energy level of the electron.
#l =# azimuthal (angular) quantum number defining the energy sub-level. Its values range from #0 " to " (n-1)#; #0=s, 1=p, 2=d, 3=f#
#m(l) =# magnetic quantum number indicating orbital within a sublevel. Range from #-l" through "#0#" to "+l#
#m(s) =# spin quantum number. It identifies an electron within an orbital and can have either of the two values #+1/2 and -1/2#
================================
The electronic configuration of calcium is:
#1"s"^2; 2"s"^2, 2"p"^6; 3"s"^2, 3"p"^6; 4"s"^2# or #["Ar"] 4"s"^2#
it is evident that the #19th# electron goes to #4"s"# energy level.
As such for it #n=4, l=0#
We know that for #l=0, m(l)=0# only.
Hence magnetic quantum number #m(l)# of #19th# electron of calcium is #=0#
