In a neutral atom, what are two particles that?
The way you worded your question, the only such atom I could think of would be one with a positron and and electron. Such an atom would quickly annihilate itself since a positron is the antimatter equivalent of the electron, and the two would attract each other and annihilate on contact, emitting a photon. I doubt that such an atom exists outside of imagination or speculation. In fact, I would say it is inconsistent with the definition of atom. But, I suspect that your question is just worded a bit awkwardly, and is really a two part question. In that case, the two particles having approximately the same mass would be the neutron and the protron. And the two particles having opposite charges would be the protron and electron. Thus the atom would consist of three atomic particles, and it would be called deuterium, an isotope of hydrogen.
