Which term refers to the number of protons in an atom and is constant for that element?

The term that refers to the number of protons in an atom, which remains constant for a specific element, is the atomic number. Each element on the periodic table is defined by its atomic number, and it uniquely identifies the element. For example, all carbon atoms have an atomic number of 6, meaning they each have 6 protons in their nuclei. This consistency in the number of protons is fundamental because it determines the element's chemical properties and its place in the periodic table.

In contrast, mass number refers to the total number of protons and neutrons in an atom's nucleus, which can vary among isotopes of the same element. An isotope specifically refers to variants of the same element that have the same number of protons but different numbers of neutrons, resulting in different mass numbers. Ionic charge relates to the net electrical charge of an atom after it has gained or lost electrons, and is not constant for a given element, as it can vary based on the atom’s ionization state.