The quantity of protons in an element is identified by the element's?

The quantity of protons in an element is specifically identified by the atomic number. This fundamental property indicates the number of protons found in the nucleus of an atom of that element, which uniquely defines the element itself. For instance, an element with an atomic number of 6 has 6 protons, making it carbon. The atomic number is crucial in organizing elements within the periodic table, where each element is listed based on its atomic number in ascending order.

In contrast, the mass number represents the total number of protons and neutrons in the nucleus and doesn't solely indicate the number of protons. Charge, which refers to the electrical state of an atom (positive, negative, or neutral), does not provide information about proton quantity either. Density measures how compactly matter is packed in an object and is unrelated to the number of protons. Thus, atomic number is the correct and most specific identifier for the number of protons in an element.