Page "Atomic number" Paragraph 11

To do this, Moseley measured the wavelengths of the innermost photon transitions ( K and L lines ) produced by the elements from aluminum ( Z = 13 ) to gold ( Z = 79 ) used as a series of movable anodic targets inside an x-ray tube.

The square root of the frequency of these photons ( x-rays ) increased from one target to the next in a linear fashion.

This led to the conclusion ( Moseley's law ) that the atomic number does closely correspond ( with an offset of one unit for K-lines, in Moseley's work ) to the calculated electric charge of the nucleus, i. e. the proton number Z.

Among other things, Moseley demonstrated that the lanthanide series ( from lanthanum to lutetium inclusive ) must have 15 members — no fewer and no more — which was far from obvious from the chemistry at that time.