What term refers to the number of protons in an atom's nucleus?

The term that refers to the number of protons in an atom's nucleus is called the atomic number. This fundamental concept in chemistry is crucial because the atomic number determines the identity of an element. For example, an atom with one proton is hydrogen, while an atom with six protons is carbon. The atomic number not only specifies the type of element but also plays a vital role in the element's chemical properties and its position in the periodic table.

In contrast, the mass number represents the total number of protons and neutrons in the nucleus of an atom, which is different from the atomic number that counts only protons. Molecular weight and atomic mass refer to the average mass of an element's atoms and the mass of a single atom respectively, but neither directly identifies the number of protons. Therefore, atomic number is the precise term that highlights the number of protons in the nucleus.