What does atomic number refer to in an atom?

Atomic number specifically refers to the number of protons found in the nucleus of an atom. This is a fundamental property of an element because it defines the element itself. For example, hydrogen has an atomic number of 1, indicating that it has one proton, while oxygen has an atomic number of 8, meaning it contains eight protons.

The atomic number is crucial in determining the chemical behavior of the element, as it influences how the atom interacts with others and the types of bonds it can form. Additionally, the atomic number is unique to each element, which helps in organizing the periodic table and identifying elements.

In contrast, the other options refer to different atomic properties: the total number of neutrons contributes to the atomic mass but does not define the atomic number, the number of electrons can vary in ions and does not equal the atomic number in neutral atoms, and the mass of the nucleus is related to both protons and neutrons but does not pertain to the atomic number specifically. Thus, the correct understanding of atomic number is pivotal in the study of chemistry and atomic structure.