Which particle is not considered in the calculation of atomic mass?

The calculation of atomic mass primarily involves the contributions of protons and neutrons, as they reside in the nucleus and account for nearly all of the atom's mass due to their substantial mass compared to electrons. Protons and neutrons are both nucleons, and their masses are used to determine the atomic mass unit (AMU) of an element.

Electrons, though present in atoms and contributing to the overall structure and charge balance, have a negligible mass compared to protons and neutrons. Their mass is so small (approximately 1/1836 of a proton) that it is often ignored in atomic mass calculations for practical purposes.

The term "nucleus" refers to the central part of the atom that contains the protons and neutrons. When considering atomic mass, the nucleus itself as a complete entity is not directly factored into the calculations; rather, it is the individual contributions of the protons and neutrons that matter. Since the question specifically asks for the particle not considered in atomic mass calculation, the nucleus fits this description. It is the constituents of the nucleus (the protons and neutrons) that determine the mass, not the nucleus as a whole.