The internal structure of an atom is called the atomic nucleus. The nucleus is located at the center of the atom and contains protons, which have a positive charge, and neutrons, which are neutral. Together, protons and neutrons make up the bulk of an atom’s mass. Surrounding the nucleus are electrons, which are negatively charged and occupy various energy levels or shells around the nucleus. The arrangement of these particles determines the chemical properties of an element, as well as its behavior during chemical reactions and interactions with other atoms.