Which subatomic particle does not contribute significantly to an atom's mass?

Protons are substantial contributors to an atom's mass, each having a mass of approximately 1 atomic mass unit (amu). Since protons reside in the nucleus alongside neutrons, they play a critical role in determining the mass of the atom. Consequently, protons are essential for the overall mass measurement of an atom.

Neutrons also contribute significantly to an atom's mass, with a mass similar to that of protons, approximately 1 amu. Like protons, neutrons are found in the nucleus and are crucial for the mass of the atom. The presence of neutrons affects both the mass number and the stability of the nucleus, further underscoring their importance in mass contribution.

This choice is misleading because while protons and neutrons do contribute significantly to an atom's mass, electrons contribute a negligible amount due to their much lighter mass. Therefore, stating that all particles contribute significantly to the atom's mass is inaccurate, as it overlooks the minimal impact of electrons.