Atomic structure refers to the arrangement of protons, neutrons, and electrons within an atom. At the center of an atom is a nucleus, which contains protons and neutrons. Surrounding the nucleus are electrons, which orbit in specific energy levels or shells. The number of protons in the nucleus determines the element's atomic number, while the number of protons and neutrons together determine the atom's mass number. The arrangement of these particles within an atom determines its chemical properties and behavior.