For example, 6 is always carbon, 92 is always uranium, and 1 is always hydrogen. Atomic number should not be confused with mass number. This page defines atomic number and mass number of an atom. All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Atomic number, 86, Relative atomic mass, . State at 20°C, Gas, Key isotopes, 211Rn, 220Rn, 222Rn. Electron configuration, [Xe] 4f145d106s26p6.