The center of an atom is called the nucleus. The nucleus is a small, dense region located at the center of an atom, and it contains most of the atom's mass. It is composed of positively charged particles called protons and electrically neutral particles called neutrons. Around the nucleus, negatively charged particles called electrons move in specific energy levels or orbitals, forming the electron cloud. The nucleus, specifically the protons it contains, determines the identity of the atom, as different elements have different numbers of protons in their nuclei.