If n=2, and l=0, then what are the possible values of ml?

The answer is 0. 

n is the principal quantum number and l is the angular momentum quantum number. The possible values of ml (magnetic quantum number) range from -l to +l. In this case, 0 is the only possible value of ml. 

But, as a further example, if n=2 and l=1, the possible values of ml would be -1, 0, and +1. 

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