How to Find How Many Protons, Neutrons & Electrons Are in Isotopes

By Emma Francesca; Updated April 24, 2017
Use the periodic table to help find information about an isotope.

An atom is composed of a nucleus and electrons orbiting around it. The nucleus itself contains protons and neutrons (with the exception of protium, an isotope of hydrogen with only a proton in the nucleus). Each element contains a specific and unique number of protons, but the number of neutrons can vary. An element, therefore, can have several variants, called isotopes, which differ slightly in the composition of the nucleus. The number of electrons can also change in an atom, giving us positive or negative ions.

Use the periodic table to find the atomic number, or number of protons, of the element you are interested in. This is the number of protons the isotope has, since the element and all its isotopes have the same atomic number. In the periodic table, the atomic number is the main number written somewhere next to the chemical symbol of the element.

Locate the mass number (the sum of the protons and neutrons) of the isotope. The mass number is usually written right after the name of the element (nitrogen-13, for example) or to the left of the chemical symbol for the element as a superscript (with the atomic number sometimes written below it).

Subtract the atomic number from the mass number. The result gives you the number of neutrons in the isotope.

Look to see if the isotope has a charge, denoted by a superscript number and a plus or minus sign next to it.

Subtract the charge from the atomic number if the charge is positive. The result is the number of electrons in the isotope.

Add the absolute value of the charge to the atomic number if the charge is negative. The result is the number of electrons in the isotope.

About the Author

Emma Francesca has a Bachelor of Science in biology and has been involved in tutoring students and writing educational materials throughout her college career. She is also a working artist with over a decade of experience with numerous art materials and techniques. She has been writing online professionally since 2011.