Question

How do you determine the possible quantum numbers for an atom?

Answer 1

The quantum numbers can be used to describe a specific electron in an atom of that element. There are four quantum numbers:

n, the principal quantum number, which describes the shell of an electron;l, the Azimuthal quantum number, which describes the subshell of an electron;ml, the magnetic quantum number, which describes/specifies the orbital an electron is in; andms, the spin quantum number, which describes the spin of the electron.

For example, if I was told that I had quantum numbers for a valence shell electron in the ground state for nitrogen were n=2,l=1,ml=−1 and ms=+12 then:

The n=2 tells me that we are describing a shell 2 electron.the l=1 describes the subshell as p. 
I now know we are talking about a shell 2, p subshell energy level for the electron.The ml=−1 would indicate the specific orbital in the p-subshell (there are three possible p-orbitals in this p-subshell with ml values = -1, 0, +1)We have specified the first 2p-subshell orbital with the ml value = -1. The spin quantum number of +1/2 indicates one of two possible spin quantum numbers (+1/2 and -1/2) for the two electrons that can fill this specific orbital in this specific subshell and shell.