Atoms are made up of protons, neutrons, and:

Atoms consist of a central nucleus, which contains protons and neutrons, surrounded by electrons. Protons carry a positive charge, while neutrons have no charge, and together they make up the nucleus of the atom. Electrons, which are negatively charged, orbit the nucleus in various energy levels or shells.

Understanding the structure of the atom is fundamental in fields such as chemistry, physics, and radiology. The arrangement of electrons plays a critical role in chemical bonding and reactions, as well as the stability of atoms. In radiological contexts, electrons are involved in interactions with radiation, affecting how materials absorb or emit energy, which is crucial for safety and regulatory practices.

The other options do not accurately represent the basic constituents of an atom. The nucleus is a part of the atom but not a component like protons or electrons. Ions are charged atoms or molecules resulting from the loss or gain of electrons but are not one of the primary building blocks of an atom. Photons are particles of light associated with electromagnetic radiation and do not constitute atomic structure.