Claim: In their atomic nuclei carbon all has the same number of protons.

The atomic number or proton number -LRB- symbol Z -RRB- of a chemical element is the number of protons found in the nucleus of an atom . It is identical to the charge number of the nucleus . The atomic number uniquely identifies a chemical element . In an uncharged atom , the atomic number is also equal to the number of electrons .   The atomic number Z , should not be confused with the mass number A , which is the total number of protons and neutrons in an atom . This number of neutrons , N , completes the weight : A = Z + N -LRB- all are whole numbers -RRB- . Since protons and neutrons have approximately the same mass -LRB- and the mass of the electrons is negligible for many purposes -RRB- and the mass defect of nucleon binding is always small compared to the nucleon mass , the atomic mass of any atom , when expressed in unified atomic mass units -LRB- making a quantity called the `` relative isotopic mass '' -RRB- , is roughly -LRB- to within 1 % -RRB- equal to the whole number A.   Atoms with the same atomic number Z but different neutron numbers N , and hence different atomic masses , are known as isotopes . A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes -LRB- see monoisotopic elements -RRB- , and the average isotopic mass of an isotopic mixture for an element -LRB- called the relative atomic mass -RRB- in a defined environment on Earth , determines the element 's standard atomic weight . Historically , it was these atomic weights of elements -LRB- in comparison to hydrogen -RRB- that were the quantities measurable by chemists in the 19th century .   The conventional symbol Z comes from the German word Zahl meaning number/numeral/figure , which , prior to the modern synthesis of ideas from chemistry and physics , merely denoted an element 's numerical place in the periodic table , whose order is approximately , but not completely , consistent with the order of the elements by atomic weights . Only after 1915 , with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms , did the word Atomzahl -LRB- and its English equivalent atomic number -RRB- come into common use in this context .