The atomic numbers on the periodic table are based on experimental observations and the understanding of atomic structure. The concept of atomic number was proposed by Henry Moseley, a British physicist, in 1913.
Moseley conducted experiments to investigate the X-ray spectra of different elements. He found that the X-ray frequencies emitted by each element were related to a fundamental property of the atom. Moseley discovered that the X-ray frequency was directly proportional to the square of the element's atomic number rather than its atomic mass.
By arranging the elements in order of increasing X-ray frequency, Moseley developed a new method of ordering the elements that aligned with their chemical and physical properties. This ordering, known as the periodic table, grouped elements with similar properties together.
Moseley's work led to the realization that the atomic number, representing the number of protons in an atom's nucleus, determined an element's identity and characteristics. The atomic number became the basis for the modern periodic table, and it serves as a unique identifier for each element, organizing them systematically and allowing for predictions of their properties.