Isotope Notation


Subscripts and superscripts can be added to an element’s symbol to specify a particular isotope of the element and provide other important information. The atomic number is written as a subscript on the left of the element symbol, the mass number is written as a superscript on the left of the element symbol, and the ionic charge, if any, appears as a superscript on the right side of the element symbol. If the charge is zero, nothing is written in the charge position. If the charge is +1 or −1, the convention is to write + or − (without the 1) as a superscript on the right. If the charge is +2, +3, −2, or −3, we write 2+, 3+, 2−, or 3− as the superscripts. 


Examples are below.

Most abundant hydrogen isotope 


Most abundant isotope of uranium


A sodium cation, Na+ 


An aluminum cation, Al3+  


An iodine anion, I

An oxygen anion, O2 

Because all of the isotopes of an element have the same atomic number, the atomic number is often left off the isotope notation. Another way of naming isotopes uses the name of the element followed by the isotope’s mass number. For example, carbon-14 can be described in two ways:


All isotopes of an element have essentially the same chemical characteristics, and there is usually no need for the chemist to distinguish between them, but sometimes the differences between isotopes are very important. For example, although the iodine atoms found in nature are almost 100% iodine-127, iodine-131 can be formed in nuclear reactions. A major difference between these isotopes is that iodine-127 atoms are stable, and atoms of iodine-131 are unstable and undergo radioactive decay. Because these isotopes have virtually the same chemical properties, -1 ions of each are absorbed by our thyroid glands in the same way (thyroid tissue specifically absorbs and stores iodine, whereas other body tissues do not). A physician who suspects that a patient has a malfunctioning thyroid gland may perform a diagnostic test in which a very small amount of sodium iodide made with iodine-131 is administered. Instruments for detecting the levels and locations of the resulting radioactive emissions can then be used to study the thyroid gland’s activity.