# What are elements?

And elements are substances that contain the same number of nuclear protons, and are characterized by the atomic number $Z$, which gives this number. $Z = 1$, we got hydrogen, $Z = 2$, we got helium, ................$Z = 26$, we got iron. You don't have to learn these numbers (but you do have to know what they represent); look at the Periodic Table..........