Medical College Admission Test (MCAT) General Chemistry Practice Exam

The mass number of an element is defined as the total count of protons and neutrons in the nucleus of an atom. This is because the mass number reflects the total mass of the nucleus, since protons and neutrons contribute the majority of the atomic mass.

In more detail, the mass number is an integer value that represents how heavy an atom is, which comes from adding together the number of protons (which identify the element) and the number of neutrons (which can vary among isotopes of the same element). Therefore, the correct interpretation of the mass number is that it is equal to the sum of these two types of particles.

This understanding is critical for grasping concepts in atomic structure and nuclear chemistry, as well as for determining isotopes—atoms that have the same number of protons but a different number of neutrons, which results in different mass numbers.