Atomic structure refers to the arrangement of particles within an atom, which is the basic unit of matter. Atoms consist of three main types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are found in the nucleus at the center of the atom, while electrons orbit around the nucleus in various energy levels. The number of protons in an atom determines its atomic number and defines the element, such as hydrogen or oxygen. Neutrons contribute to the atomic mass, and the balance of protons and electrons determines the atom's charge, leading to neutral or charged particles called ions.