At the center of an atom resides the nucleus, which is composed of protons and neutrons. Protons are positively charged particles, while neutrons are neutral, contributing to the atom's mass. The nucleus contains almost all of the atom's mass, while electrons, which are negatively charged, orbit around the nucleus in various energy levels or shells. The number of protons in the nucleus determines the atomic number and identifies the element, while the arrangement of electrons affects how the atom behaves chemically. The interactions between these particles govern the atom's properties and reactions with other atoms.