Given #Z#, #"an atomic number"#, how do I know which element is referenced?
And have you got a Periodic Table in front of you...........?
See if this older answer and links address your question. You should not have to remember this Periodic Table, tho' you soon remember the atomic numbers (and masses) of common elements, i.e.
Now in fact a Periodic Table should be supplied to you in EVERY test in Chemistry and Physics that you ever sit. It is up to you to learn how to use it. Go for it.