Kaplan Nursing Entrance Practice Exam

What is the mass number of an element?

1. Number of Electrons and Neutrons

2. Number of Protons only

3. Number of Protons and Neutrons

4. Number of Nuclei

The correct answer is: Number of Protons and Neutrons

The mass number of an element is the total number of protons and neutrons found in the nucleus of an atom. Since protons and neutrons together contribute to the overall mass of the atom, their combined count gives you the mass number. Electrons, although they are also a fundamental part of atoms, have a negligible mass compared to protons and neutrons, so they do not factor into the mass number. The number of nuclei as a choice is incorrect as it does not apply to the definition of the mass number in the context of an individual atom. This understanding is critical for studying atomic structure and how elements interact in various chemical processes.