The atomic number is denoted by the symbol "Z" because it comes from the German word "Zahl," which means "number." The term "Zahl" was first used by the German chemist Julius Lothar Meyer in the late 19th century when he was working on the periodic table of elements.
Meyer, along with Dmitri Mendeleev, independently developed a system for organizing the elements based on their chemical properties and atomic weights. Mendeleev's periodic table, which gained more recognition, arranged the elements in order of increasing atomic weights and grouped them based on their similar chemical properties.
Meyer, on the other hand, organized the elements by their valence (the combining capacity of an atom) and noticed a periodic pattern in their properties. He assigned a unique number to each element to represent its position in the periodic table. This number, which represented the element's position in the periodic table and corresponded to its atomic weight, was denoted by the letter "Z."
Later, with the advancement of atomic theory and the discovery of the atomic structure, it was understood that the atomic number actually represented the number of protons in an atom's nucleus. However, the term "Z" for atomic number has persisted in scientific literature and is still widely used today.