Other subatomic particles called neutrons also exist. Protons and neutrons, together, comprise the atomic nucleus. The atomic nucleus is composed of positively charged protons and electrically neutral neutrons. The number of protons in the nucleus of an atom defines an element. The number of neutrons defines the isotope of an element.
Isotopes are atoms of the same element that have different number of neutrons within their nuclei.
The number of protons and electrons are the same across a series of isotopes, however the number of neutrons change. The three most simple isotopes of the hydrogen atom are known. The naturally occuring hydrogen atom contains only a proton in its nucleus, and is called protium. The addition of one neutron changes protium to deuterium. The third isotope of hydrogen is tritium and contains two neutrons.