How is the atomic number defined?

The atomic number is defined as the number of protons in the nucleus of an atom. This fundamental characteristic uniquely identifies an element and determines its position on the periodic table. Each element has a distinct atomic number, which is critical in defining its chemical properties and behavior.

For instance, hydrogen has an atomic number of 1 because it has one proton, while carbon has an atomic number of 6 due to its six protons. The atomic number is essential not only for differentiation between elements but also plays a crucial role in nuclear reactions and the chemical bonding processes that occur between different elements.

In contrast, the other choices describe different concepts: the number of neutrons (which determines isotopes of an element), the total number of protons and neutrons (which gives the mass number), and the mass number itself (the total count of protons and neutrons combined). None of these definitions reflect the specific definition of atomic number. Understanding the atomic number's definition helps clarify elemental identity and behavior in various scientific contexts.