Here's why:
* amu (atomic mass unit): This unit is defined as 1/12th the mass of a carbon-12 atom.
* Proton: Protons are found in the nucleus of an atom and have a positive charge. Their mass is very close to 1 amu.
* Neutron: Neutrons are also found in the nucleus and have no charge. Their mass is slightly larger than a proton, at about 1.0087 amu.
* Electron: Electrons orbit the nucleus and have a negative charge. Their mass is much smaller than protons and neutrons, about 0.00055 amu.
Therefore, while neutrons are also close to 1 amu, the proton is the subatomic particle with the closest mass to 1 amu.
