Quantum numbers are n, l, m(sub.l), m(sub.s). Confusing, but heres the gist of it:

n represents your row number. Chlorine is in the 3rd row, so n=3. Easy.

"L" can be a range of numbers from 0 to one less than n (n=3, so L can range from 0 to 2). These numbers (0,1,2) represent the possible shells a valence chlorine electron can reside. 0=s 1=p, 2=d. Since we're dealing with the 13th electron, we're in a P orbital (so n=1)

"MsubL" represents the possible sub-shells the valence chlorine electron can reside. Numerically, MsubL=+ or -(L). Since we're looking at a P orbital, L=1. Thus, each sub-shell correlates to -1, 0, and 1. Then we fill the subshells with 1 electron each before doubling up (sounds familiar?). Since the 13th electron is the only electron in our P orbital, its gonna end up in the -1 sub-shell. So... MsubL=-1.

MsubS is easy. Its just the electron spin (what are thooseee). Who knows, and who cares. Just remember its either +1/2 or -1/2.

So, to summarize:

N = 3

L = 1

MsubL = -1

MsubS = +/- 1/2

Pretty sure you knew all this, but I'm just covering bases here. And heres my point:

**Note how L and MsubL would have been completely different if we were asked to solve for the 11th or 12th electron. **

Good luck.