The atomic number, #Z#, specifies the number of massive, positively charged particles present in the element's nucleus, and this number defines the identity of the element: #Z=1#, hydrogen, #Z=2#, helium, ...... #Z=6#, carbon........#Z=31#, gallium, etc.
Because elements, (and all matter) are usually neutral entities, if there are #Z# positive charges, there must be #Z# negative charges in the neutral atom. In other words, #Z="number of electrons"# for the neutral element.
If #Z=11#, then what is the element? This element has an atomic mass of #23*"amu"#. How do we account for the mass excess to that of the protons? Certainly electrons don't contribute any mass.
