Which particles consist of two protons and two neutrons?

The particles that consist of two protons and two neutrons are known as alpha particles. This configuration reflects the nucleus of a helium atom, as an alpha particle is essentially the same as a helium-4 nucleus. The presence of two protons accounts for the positive charge, while the two neutrons contribute to the overall mass without affecting the charge.

Alpha particles are commonly emitted during radioactive decay processes, particularly in the decay of heavy elements. Their relatively large mass and positive charge mean that they have limited penetration ability compared to other types of radiation (like beta or gamma radiation). This property makes alpha radiation less hazardous externally; however, it can be harmful if alpha-emitting materials are ingested or inhaled.

In contrast, beta particles are high-energy, high-speed electrons or positrons emitted during certain types of radioactive decay, while gamma radiation consists of high-energy photons that carry no charge or mass. Delta particles are not a standard category in nuclear physics as the other three are, which makes them unrelated to the composition discussed in this question. Thus, the unique characteristics of alpha particles, defined by their structure of two protons and two neutrons, correctly identify the answer.