The term "atomic number" is used instead of "proton number" because it refers to a fundamental property of atoms as a whole, not just the number of protons. The atomic number represents the unique identity of an element and is directly related to the number of protons in the nucleus of an atom. However, it also has broader implications in terms of the element's chemical properties and its position in the periodic table.
The concept of atomic number emerged during the development of the periodic table by Dmitri Mendeleev and others. They noticed a recurring pattern in the chemical properties of elements when arranged in increasing order of atomic number. This pattern could not be explained solely by the number of protons but required a more comprehensive understanding of the atom's structure and behavior.
By using the term "atomic number," scientists emphasize that it is a fundamental property that encompasses the overall structure and behavior of an atom. It encompasses the number of protons, which define the element's identity, but also relates to other important aspects such as electron configuration, valence electrons, and chemical reactivity.