The particle that contributes a negligible amount to the atomic mass is the electron.

Here's why:

* Protons and Neutrons: These particles reside in the nucleus of an atom and have almost identical masses. They are the primary contributors to the atomic mass.

* Electrons: Electrons orbit the nucleus and are significantly smaller than protons and neutrons. Their mass is approximately 1/1836th the mass of a proton or neutron.

Therefore, when calculating the atomic mass, the mass of electrons is usually disregarded because it is so small compared to the mass of protons and neutrons.