Example Problem: Isotopes and Nuclear Symbols

How to Write the Nuclear Symbol of an Element

The nuclear symbol for an element, such as oxygen, includes information about the number of protons and neutrons in the atom.
The nuclear symbol for an element, such as oxygen, includes information about the number of protons and neutrons in the atom.

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This worked problem demonstrates how to write nuclear symbols for isotopes of a given element. The nuclear symbol of an isotope indicates the number of protons and neutrons in an atom of the element. It does not indicate the number of electrons. The number of neutrons is not stated. Instead, you have to figure it out based on the number of protons or atomic number.

Nuclear Symbol Example: Oxygen

Write the nuclear symbols for three isotopes of oxygen in which there are 8, 9, and 10 neutrons, respectively.


Use a periodic table to look up the atomic number of oxygen. The atomic number indicates how many protons are in an element. The nuclear symbol indicates the composition of the nucleus. The atomic number ( the number of protons) is a subscript at the lower left of the symbol of the element. The mass number (the sum of protons and neutrons) is a superscript at the upper left of the element symbol. For example, the nuclear symbols of the element hydrogen are:

11H, 21H, 31H

Pretend that the superscripts and subscripts line up on top of each other: They should do it this way in your homework problems, even though it's not printed that way in this example. Since it's redundant to specify the number of protons in an element if you know its identity, it's also correct to write:

1H, 2H, 3H


The element symbol for oxygen is O and its atomic number is 8. The mass numbers for oxygen must be 8 + 8 = 16; 8 + 9 = 17; 8 + 10 = 18. The nuclear symbols are written this way (again, pretend the superscript and subscript are sitting right on top of each other beside the element symbol):

168O, 178O, 188O

Or, you could write:

16O, 17O, 18O

Nuclear Symbol Shorthand

While it's common to write nuclear symbols with the atomic mass—the sum of the number of protons and neutrons—as a superscript and atomic number (the number of protons) as a subscript, there's an easier way to indicate nuclear symbols. Instead, write the element name or symbol, followed by the number of protons plus neutrons. For example, helium-3 or He-3 is the same as writing 3He or 31He, the most common isotope of helium, which has two protons and one neutron.

Example nuclear symbols for oxygen would be oxygen-16, oxygen-17, and oxygen-18, which have 8, 9, and 10 neutrons, respectively.

Uranium Notation 

Uranium is an element often described using this shorthand notation. Uranium-235 and uranium-238 are isotopes of uranium. Each uranium atom has 92 atoms (which you can verify using a periodic table), so these isotopes contain 143 and 146 neutrons, respectively. Over 99 percent of natural uranium is the isotope uranium-238, so you can see that the most common isotope isn't always one with equal numbers of protons and neutrons.