Henry Moseley's discovery of the atomic number refined the way elements are listed in the periodic table. Prior to his discovery, the elements were listed by atomic mass. Chemists considered the atomic numbers to be somewhat arbitrary rather than concrete.
Henry Moseley was an English physicist born in 1887. After studying chemistry and physics at Oxford's Trinity College, he joined Ernest Rutherford's research group at Manchester College. While working in this group, Moseley created the first atomic battery.
From there, he became interested in x-rays and how they worked. He began to study them in order to determine how the inside of atoms worked. Moseley wanted to prove the hypothesis that the atomic number is equal to the amount of the charge in the atom's nucleus. Until that hypothesis was proven, the atomic number was simply the numerical representation of an element's position in the periodic table.
After working through a number of different experiments, Moseley determined that the number of protons was the basic difference between elements. This discovery proved the hypothesis of the atomic number and, therefore, helped to refine the order of elements in the periodic table. After rearranging the elements according to their atomic number rather than the atomic weights, fundamental flaws in the original periodic table were revealed.