What are atomic numbers and isotopes?

#Z# defines the identity of the element. If #Z=1#, then the element is hydrogen, if #Z=2#, then the element is helium, #Z=3#, then the element is lithium,.......#Z=55#, then the element is caesium.

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 #Z=1# to #Z=100#. Of what element do we speak if #Z=100#?

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, #Z=1#, there are normally no neutrons. Some small percentage of hydrogen atoms contain 1 neutron, the isotope deuterium. We would represent this as #""^2H#; the symbol #H# specifies that #Z=1# by definition; the superscripted mass number gives the number of neutrons, AFTER we subtract #Z#. A smaller percentage of hydrogen atoms contain 2 neutrons, the isotope tritium, #""^3H#. How many neutrons in tritium?

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.