What is an Isotope?

What Are Isotopes?
Hi, and welcome to this video on Isotopes.

Let’s jump right in with a definition: Isotope = various forms of the same element that have an equal number of protons (and therefore the same atomic number) but differing numbers of neutrons in their nuclei, and therefore have different atomic mass but not chemical properties (a common example would be a radioactive form of an element). As of now, all 81 elements have at least one isotopic form. There are currently 275 isotopes of the 81 stable elements, which does not include the over 800 unstable and radioactive isotopes.

Let’s take a look at some examples.


– Carbon atoms exist naturally with 6, 7 or 8 neutrons. Since each atom of carbon has 6 protons, the isotopes must have atomic masses of 12, 13 and 14. (Since atomic mass = mass protons + mass neutrons)

– These isotopes are called carbon-12, carbon-13 and carbon-14. Alternatively, they may be written 12C, 13C and 14C.

– Carbon-12 and carbon-13 are stable. Carbon-14 is unstable, decaying with a half-life of about 5,700 years. It is produced in earth’s atmosphere by cosmic ray bombardment of nitrogen-14.

Mass = superscript and atomic number is subscript
We can express the Carbon isotope

        12C   P=6 N=6 Mass=12
13C P=6 N=7 Mass=13


Hydrogen is unique in that it’s the only element whose isotopes have alternative names and who start out with no neutrons.

     - Hydrogen -1: 1 Proton, 1 Electron, 0 Neutron
Atomic number: atomic mass:
- Hydrogen -2: 1 proton, 1 electron, 1 neutrons
Atomic number: atomic mass:
- Hydrogen-3: 1 proton, 1 electron, 2 neutrons
Atomic number: atomic mass:

I hope that helps, thanks for watching this video lesson, and until next time, happy studying.



by Mometrix Test Preparation | Last Updated: August 15, 2019