The atomic number of an atom represents the number of protons found in the nucleus of that atom. Elements are identified by their atomic numbers because each element has a different number of protons in its nucleus.
The periodic table of the elements is organized by atomic numbers. The first element in the periodic table is hydrogen, which has an atomic number of one and therefore only one proton in its nucleus. The periodic table ends with ununoctium, a highly unstable synthetic element with an atomic number of 118. As science progresses, it may become possible to synthesize elements with even higher atomic numbers, but the dangerous instability of these elements makes it difficult to create new elements.