The core, or dense center of an atom, is called the atom's nucleus. The nucleus contains protons, which have positive charges, and neutrons, which have no charge and are neutral.
Each proton and neutron in an atom's nucleus has approximately the same mass as each other. The number of protons in the nucleus determines the element to which an atom belongs, and the number of neutrons determines its isotope. Atoms also contain electrons, which travel outside of the nucleus. Although electrons have much smaller masses than protons and neutrons, each electron has a negative charge of the same magnitude as the positive charge carried by a proton.