Explain why the quantum number set (2, 2, -1, -½) is not possible for an electron in an atom.

Question

thadyoung · Answer

What you have here are: $\sf (n, l, m_l, m_s)$
In order to understand WHY you cannot have this arrangement of quantum numbers, you need to be familiar what each of them are. So i ask, do you know what each of them are?
n = principal quantum number
l = angular momentum quantum number
m$_l$ = magnetic quantum number
m$_s$ = magnetic spin 

$l$ depends on n.
To find $l$, you need to have: $\sf \color{red}{n-1} =$ $l$
this is what determines the orbital shape! (the shbshells) This is all you need to know, in general. 
That the values can range from 0 to n-1.