An atom consists of a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge. The total number of protons in an atom determines its atomic number and its place on the periodic table. The number of electrons in an atom is usually equal to the number of protons, so most atoms are neutral. However, if an atom gains or loses electrons, it becomes charged and is called an ion. A positively charged atom is called a cation, and a negatively charged atom is called an anion.