Kaplan Nursing Entrance Practice Exam

What is the mass number of an atom?

1. Number of electrons

2. Number of protons only

3. Number of protons and electrons

4. Number of protons and neutrons

The correct answer is: Number of protons and neutrons

The mass number of an atom is determined by the sum of the number of protons and neutrons in its nucleus. Protons, which carry a positive charge, are fundamental to defining the identity of an element, while neutrons, which are neutral, contribute to the atom’s mass but do not affect its chemical properties directly. This means that for any given atom, the mass number provides an indication of its overall size and mass based on the total amount of nuclear particles that determine its structure. While the number of electrons does play a role in the atom's charge and chemical behavior, it does not contribute to the mass number, as electrons are significantly lighter than protons and neutrons. In contrast, merely counting protons or bundling protons with electrons would not yield the mass number. Thus, for an accurate representation of mass, the addition of neutrons is essential, confirming that the correct answer reflects this fundamental principle of atomic structure.