The concept of atomic number and the understanding of the order of elements on the periodic table developed through the work of several scientists, including Dimitri Mendeleev, Julius Lothar Meyer, and Henry Moseley.
In the 19th century, chemists were studying the properties of elements and organizing them based on their characteristics. In 1869, Mendeleev and Meyer independently proposed their versions of the periodic table. They both noticed a periodic repetition of properties when elements were arranged in order of increasing atomic weights.
Mendeleev arranged the elements in a table based on their atomic weights, leaving gaps for elements that were yet to be discovered. He observed that elements with similar properties tended to occur at regular intervals and predicted the existence and properties of some missing elements. Mendeleev's periodic table became widely accepted.
In the early 20th century, Henry Moseley conducted X-ray experiments on different elements and found that the X-ray spectra of elements were related to their atomic numbers rather than their atomic weights. He discovered a direct relationship between the wavelength of the X-rays emitted by an element and its atomic number.
Moseley's work confirmed that the atomic number, representing the number of protons in an atom's nucleus, was a fundamental property of an element. It provided a more accurate and logical way to arrange the elements on the periodic table. The periodic table was then reorganized based on atomic numbers, with elements arranged in order of increasing atomic number rather than atomic weight.
Through a combination of experimental observations, data analysis, and theoretical insights, scientists were able to establish the concept of atomic number and determine the specific atomic numbers of elements based on their properties and spectroscopic studies. This understanding laid the foundation for the modern periodic table we use today.