Neutrons.
The nucleus of an atom contains two kinds of particles, protons and neutrons. The number of protons determines what element the atom is. Atoms that have the same number of protons but different numbers of neutrons are isotopes of the same element.
For example, if two atoms both have 1 proton in their nuclei, but one atom has 0 neutrons and the other has 1 neutron, both atoms are hydrogen but they are different isotopes of hydrogen.
