ASNT Industrial Radiography Radiation Safety Practice Test

What is the atomic weight of an atom defined as?

1. The total number of neutrons only

2. The combination of protons and electrons

3. The combination of the total number of protons and neutrons in the nucleus

4. The total mass of all atomic particles

The correct answer is: The combination of the total number of protons and neutrons in the nucleus

The atomic weight of an atom is defined as the combination of the total number of protons and neutrons in the nucleus. This is because atomic weight, often referred to as atomic mass, typically reflects the mass number of an atom, which is calculated by adding the number of protons (which define the element) and the number of neutrons (which contribute to the mass but not to the charge). Protons and neutrons reside in the nucleus of the atom and have a significant contribution to its overall mass. Electrons, while they contribute to the overall size of the atom and its charge, have a much smaller mass compared to protons and neutrons and are generally not included in the calculation of atomic weight. Thus, the correct answer emphasizes this relationship between protons and neutrons in determining atomic weight, providing a clear understanding of what constitutes the mass of an atom in nuclear chemistry and physics.