there are four quantum numbers (n, l, ml, and ms).
'n' refers to the shell (energy level) of electrons in which a particular electron might be found. as you increase in the number of electrons it becomes necessary to have more shells b/c the maximum number of electrons you can have in a shell is 8.
'l' refers to s, p, d, f, and g which are the first 5 subshell of electrons. the maximum number of electrons in a subshell = 4l + 2 (simplified s(2), p(6), d (10), f (14), and g (18)
'ml' as a quantum number refers to the specific orbital in the 'l' subshell... the numbers range from l to -l as you had mentioned which means that the p subshell (where l = 1, for example) has numbers -1, 0, and 1. Because there are three numbers, we know that the p subshell has 3 orbitals. Here is a list of the other 'ml' quantum numbers.
s 0 = 1 numer... 1 orbital
p -1,0,1 = 3 numbers... 3orbitals
d -2,-1,0,1,2 = 5 numbers... 5 orbitals
f -3,-2,-1,0,1,2,3 = 7 orbitals... are you catching on???
Here is the key to your question.... the most negative 'ml' number is the first orbital to be filled with an electron! for the d orbital ml = -2 would be the first one filled with an electron. Followed by -1, then 0, and so on. Remember that Hund's Rule states that electrons will fill to create the maximum number of half-filled orbitals... so each orbital may contain 2 electrons but all orbitals must be filled with an electron before any electrons can 'pair up'
Hope this answers your question...
