 Nowadays, chemical nomenclature, including the definition of the element, is regulated by The International Union of Pure and Applied Chemistry (IUPAC). In 1923, IUPAC followed Mendeleev and standardized the individuation criteria for the elements by explicitly endorsing the actual components thesis. Where they differed from Mendeleev is in what property they thought could best individuate the elements. Rather than using atomic weights, they ordered elements according to atomic number, the number of protons and of electrons of neutral elemental atoms, allowing for the occurrence of isotopes with the same atomic number but different atomic weights. They chose to order elements by atomic number because of the growing recognition that electronic structure was the atomic feature responsible for governing how atoms combine to form molecules, and the number of electrons is governed by the requirement of overall electrical neutrality (Kragh 2000).