What are atomic numbers and isotopes?
Do you have to remember all this? No! Because for every exam in chemistry and physics you will ever sit, you will (AND MUST) be supplied with a Periodic Table that lists the atomic numbers and masses of all known elements. There about a 100 or so, from
On the other hand, the nucleus may contain different numbers of neutrons, massive, neutrally charged nuclear particles. This explains the phenomenon of isotopes. For the element hydrogen,
Most elements have a number of isotopes. The quoted atomic mass you see on the Table is the weighted average of the individual isotopes. This generally does not affect the chemistry of the element.