Atomic number

It was now possible to measure the wavelengths of elements' characteristic X rays. It was found that the higher the weight of the element, the higher the frequency of the characteristic X ray. This led to a major improvement of the periodic table, with the amount of positive charge on the nucleus now called the atomic number.
 Henry Gwyn Jeffreys Moseley England 1914 Atomic number

It was now possible to measure the wavelengths of elements' characteristic X rays. It was found that the higher the weight of the element, the higher the frequency of the characteristic X ray. This led to a major improvement of the periodic table, with the amount of positive charge on the nucleus now called the atomic number.