A Level Chemistry OCR Practice Exam

The mass (nucleon) number is defined as the total number of protons and neutrons found in the nucleus of an atom. This is essential because both protons and neutrons contribute significantly to the overall mass of the atom, while electrons, which reside in the outer regions, have a negligible mass compared to nucleons.

Protons contribute a positive charge, and their quantity identifies the element, linking closely to the atomic number. Neutrons, on the other hand, do not carry a charge but play a critical role in the stability of the nucleus. Together, these protons and neutrons are collectively referred to as nucleons, which is why the expression "nucleon number" is synonymous with mass number. Such understanding is critical for comprehending atomic structure and behavior, particularly in nuclear chemistry and reactions.

The other definitions do not accurately represent the concept of mass number—for instance, the number of electrons equates to the electrical charge balance in neutral atoms, and defining based on protons alone fails to consider neutrons' contribution to atomic mass.