What is defined as the sum of protons and neutrons in an atom?

The mass number is the term used to represent the sum of protons and neutrons within an atom. This is a crucial concept in understanding the structure of an atom. Protons, which are positively charged, and neutrons, which are neutral, reside in the nucleus of the atom, and together they account for nearly all of the atom's mass.

The atomic number, on the other hand, refers to the number of protons in the nucleus of an atom and determines the element's identity. Molecular weight pertains to the weight of an entire molecule, which can include multiple atoms bonded together, and is not limited to just protons and neutrons in a single atom. Elemental mass is not a standard term used in atomic theory and can cause confusion as it does not directly correlate to the sum of protons and neutrons. Therefore, the correct choice captures the fundamental concept of atomic structure effectively.