Actually, Dmitri Mendeleev arranged elements in order of increasing atomic mass, not atomic number. When Mendeleev developed the periodic table in the late 19th century, the concept of atomic number was not yet understood.
Mendeleev noticed that when he arranged the elements in order of increasing atomic mass, certain patterns emerged. He observed that elements with similar chemical properties appeared at regular intervals, and he organized them into columns or groups. This arrangement allowed him to predict the properties of missing elements and even suggest the discovery of new elements that fit into the gaps in his table.
Later, in the early 20th century, Henry Moseley discovered that the properties of elements were better explained by their atomic number rather than their atomic mass. Moseley's experiments showed that the atomic number of an element, which represents the number of protons in the nucleus, is a fundamental property of each element. This discovery led to the modern understanding of the periodic table based on atomic number, as it provides a more accurate and consistent way to organize the elements according to their chemical and physical properties.