ABSTRACT
Two isotopes of a chemical element differ only in the number of neutrons in the nucleus. Hydrogen was the first discovered example. The nucleus of the most common hydrogen isotope is a single proton. The nucleus of the second hydrogen isotope, called the deuteron, contains both a proton and a neutron. One of every 6400 hydrogen nuclei is a deuteron. The extra neutron makes a deuterium atom about twice as massive as an ordinary hydrogen atom. However, the single electron for both isotopes means their chemical properties are nearly identical.
