Atomic Number

All atoms of a particular element have the same number of protons in the nucleus.  This number is a basic property of an element, called its atomic number and given the symbol Z:

Z = number of protons

In a neutral atom, the number of protons in a nucleus is exactly equal to the number of electrons outside the nucleus.  Consider, for example, the elements hydrogen (Z = 1) and uranium (Z = 92).  All hydrogen atoms have one proton in the nucleus, all uranium atoms have 92.  In a neutral hydrogen atom there is one electron outside the nucleus; in a uranium atom there are 92.

H atom:  1 proton, 1 electron                          Z=1

U atom:  92 protons, 92 electrons                   Z=92