What does the mass number of an element indicate?

The mass number of an element indicates the total number of protons and neutrons in the nucleus of an atom. This number is significant because it provides a measure of the atomic mass and reflects the most stable isotopes of that element. The mass number is calculated by adding the number of protons, which determine the atomic number and the identity of the element, to the number of neutrons, which can vary among isotopes of the same element.

Understanding the mass number is crucial for various scientific applications, including nuclear chemistry and physics, where isotopes play a vital role. This distinction between neutrons and protons is fundamental in explaining how elements exist in different forms and why some isotopes are stable while others are radioactive. The other options focus on neutrons, electrons, or protons alone, which do not convey the comprehensive understanding provided by the mass number.