In any given samples of an element there is usually a mixture of isotopes. The international atomic weight of any element is listed as an average of the weights of all the various isotopes normally (commonly) present.
For example, the weight of a single atom of normal (the most common isotope) chlorine is 35. But any considerable quantity of the element also contains about 25 percent of heavy chlorine, CI37. The international atomic weight of chlorine as an element is therefore given as 35.457.
Similarly, the atomic weight of normal hydrogen is 1 bout the international atomic weight is 1.00814. Three isotopes of hydrogen are known: H1, mass (at. wt.) 1.00814; deuterium (H2), mass 2.01474; tritium (H3), mass 3.01701. Tritium is radioactive; it is so designated by the asterisk (*).
There are isotopes of nearly every element; not all are radioactive. Chlorine has two principal (most common) isotopes with atomic weights of 35 to 37. The atomic weight of an isotope is generally indicated by a superscript number (CI35, CI37).
Lead has at least 16 isotopes, carbon five or six, and so on. In the periodic table all of the isotopes of an element are placed together, since the position of each element in the table depends only on the number of its electrons (at. no.), not its nuclear structure (at. wt.).
Isotopes of an element are chemically similar since it is mainly the electrons that determine the chemical properties. However, since isotopes have different numbers of neutrons, they may differ markedly in physical properties because these are affected by their atomic weights.
For example, of the four principal carbon isotopes C11*, C12, C13 and C14* one (C11*) is lighter and two are heavier (C13 and C14*) than the “normal” C12 carbon atom. The radioactive isotopes may be detected by means of a Geiger counter.