Question

How to determine the n,l, and ml for cr,fe and Mg?

Answer 1

See Below

Explanation:

We don't generally determine n, l, ml for an entire atom. An atom has many electrons in it, and the n, l, ml are the quantum numbers associated with a single electron. They represent something similar to the address for each electron.

So your question is sort of like asking "I have a bunch of people hanging out in IKEA - what is their home address"? They each have a home address.

We generally ask about the n, l, ml for an individual electron in one of those atoms, like the 2nd 4s electron added in Fe.

In this case, that electrons n=4, l=0, ml=0..and if you are doing ms=-1/2.