Moseley developed a special method of determining the charge of an atom’s nucleus using x-ray radiation. He applied his x-ray technique to measure the integer charge on elemental nuclei, and ordered the elements based on that number, rather than using atomic mass. When he did this, tellurium fell perfectly into place before iodine, and nickel fell perfectly into place before cobalt. He, thus, solved the riddle that had plagued Mendeleev’s table for more than 40 years.