1. A proton. A proton is a positively charged particle found in the nucleus of an atom. It contains two up quarks and one down quark, and has a mass that is approximately equal to that of a neutron. However, protons are not neutrons because they have a different electric charge.

2. An electron. An electron is a negatively charged particle that orbits the nucleus of an atom. It has a very small mass compared to a proton or a neutron, and it does not contain any quarks. Electrons are not neutrons because they have a different mass and charge.

3. A photon. A photon is a particle of light that has no mass and no electric charge. It is the quantum of the electromagnetic field, and it is responsible for all electromagnetic interactions. Photons are not neutrons because they have no mass or charge.