Protons and neutrons have almost the same mass, but the proton is slightly lighter than the neutron. This may seem contradictory to some sources that state otherwise, but it is important to consider the context in which the information is presented.
The mass of a particle is typically expressed in terms of its rest mass, which is the mass of the particle when it is at rest. The rest mass of a proton is approximately 1.6726219 x 10^-27 kilograms, while the rest mass of a neutron is approximately 1.6749275 x 10^-27 kilograms. Therefore, the neutron has a slightly greater rest mass than the proton.
However, it is important to note that in certain contexts, such as nuclear reactions or particle interactions, protons and neutrons can be in different energy states, and their masses can change due to the binding energy involved. When protons and neutrons are bound together in atomic nuclei, the overall mass of the nucleus can differ from the sum of the individual masses of the protons and neutrons. This is known as the mass defect, and it represents the energy released during the formation of the nucleus.
In some sources, you may come across statements that refer to the "effective mass" of protons and neutrons within the nucleus. This effective mass can vary depending on the specific nucleus and the energy state of the nucleons (protons and neutrons) within it. In some cases, the effective mass of protons can be greater than that of neutrons due to the specific nuclear properties and energy levels involved. However, when considering the rest masses of isolated protons and neutrons, the neutron has a slightly greater mass.
So, to summarize, the rest mass of a neutron is slightly greater than that of a proton. However, in certain contexts, such as within atomic nuclei, the effective mass of protons can be greater due to the specific nuclear properties and energy levels involved.