On the Periodic Table of elements, there are atomic numbers for each element. What is their relevence? How do you calculate them?
The atomic number.
Do you have to remember these numbers? You shouldn't, because for every examination in chemistry or physics you ever sit, you should and must be supplied with a copy of the Periodic Table as a matter of course. Every student, every physical scientist, and every engineer will have a Periodic Table on hand for reference. Where do you keep yours?