1. Education
You can opt-out at any time. Please refer to our privacy policy for contact information.

Discuss in my forum

Isotopes and Nuclear Symbols

Worked Chemistry Problems


This is a periodic table tile for the element oxygen.

This is a periodic table tile for the element oxygen.

Todd Helmenstine

This worked problem demonstrates how to write nuclear symbols for isotopes of a given element.


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 (number of protons) is a subscript at the lower left of the symbol of the element. The mass number (sum of the protons and neutrons) is a superscript to the upper left of the element symbol. For example, the nuclear symbols of the element hydrogen are:

11H, 21H, 31H

Pretend that the superscipts and subscripts line up on top of each other - they should do so in your homework problems, even though they don't in my computer example ;-)


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

©2014 About.com. All rights reserved.