How do you determine an atom's mass?

An atom's mass is primarily determined by the combined weight of its protons and neutrons. Protons and neutrons are located in the nucleus of the atom and contribute significantly to its overall mass. Each proton and neutron has approximately one atomic mass unit (amu), while electrons are much lighter and contribute negligibly to the total mass of the atom.

Adding the number of protons and neutrons gives the atomic mass number, which is a practical way to express the mass of the atom in most scientific contexts. This value is critical for understanding various chemical properties and behaviors of elements in reactions and bonding.

The other options do not provide an accurate method for determining an atom's mass. Counting electrons, for instance, does not contribute meaningfully to the mass since they are so lightweight compared to protons and neutrons. Thus, option C is the correct choice, as it directly relates to the key contributors to atomic mass.