Henry Moseley established that atomic number, not atomic weight, determines the order of the periodic table.
In 1913, Moseley studied the characteristic X-rays emitted by elements when bombarded with electrons. He found that the square root of an X-ray’s frequency followed a regular relationship with atomic number. This became known as Moseley’s law and gave atomic number a firm physical meaning: it corresponds to the positive charge of an atom’s nucleus.
Before this work, the periodic table was generally arranged by increasing atomic weight, although chemists had to make exceptions when chemical properties demanded them. Moseley’s measurements explained puzzles such as cobalt appearing before nickel even though cobalt has a slightly greater atomic mass.
Moseley did not invent every part of the atomic-number concept. Ernest Rutherford and Antonius van den Broek had proposed related nuclear ideas, while Dmitri Mendeleev created the earlier periodic framework. Moseley’s decisive contribution was experimental proof that the table’s sequence follows nuclear charge.