More Than One Version Of Each Element

The Atomic Mass Mystery


Isotopes exist as the same element but with different mass numbers. All nuclei of any element must contain the same number of protons. They have different masses, because each isotope has a unique number of neutrons.


Examine the entries for each element on the periodic chart and you’ll see the atomic mass of the elements include some fraction smaller than one. If neutrons and protons have masses of approximately one, alone it makes no sense to find fractions of one included in the mass. 



Figure 1 shows an entry for carbon. Instead of finding 12.00 amu as you expect, the entry has 12.01. This results from the fact the atomic mass is composed of carbon atoms which have a mass of 12.00 amu and 13.00 amu . The atomic mass results from the weighted average  of the carbon with a mass number of 12.00 and 99% abundant and carbon with a mass number of 13.00. 


⦿ definition

Isotopes are atoms of the same element which have different mass numbers




carbon on periodic table
Figure 1: Essential information for each element
  • Atomic Mass

Weighted average mass of all the isotopes of a given element


  • Mass Number

Mass of single kind of isotope 

One Size Fits All


Isotopes exist as atoms of the same element. Each isotope has a different total mass. Even though all isotopes of a single element must have the same number of protons, they differ from one another by the number of neutrons.


Consider only the nucleus when you are concerned with isotopes. Electrons play no part in isotopes or the atomic mass, Figure 2. Only protons and neutrons contribute to the mass of an isotope.




Focus on nucleus of atom
Figure 2: Focus on nucleus contains protons and neutron

Isotopes Are Useful


You find isotopes in many roles when they are put to use:


  • BNCT


Boron neutron capture therapy is used to treat tumors which might otherwise be difficult to operate on. It relies on 10B capturing neutrons. The new unstable 11B undergoes α-decay. Then it produces a 7Li ion. The α-particle attacks the cancer cell form within the cell.



  • Radiometric Dating


The most important radiometric dating is carbon dating. It relies on the fact 14C decays to 14N, which has a half-life of about 5,200 years. Carbon dating becomes ineffective earlier than 50,000 years in the past.


  • Nuclear Energy


The most common isotopes used to produce nuclear energy are 235U and Pu239. Both must be extracted from the population of other isotopes of the same element before a sample becomes useful for nuclear reactions.



  • Medical Imaging


99Te is the most common radio nuclide used for radioactive tracers. It finds use to examine brain function without resort to invasive methods.


This list is not exhaustive, but just the tip of the iceberg. It is intended to illustrate a variety of contributions made by isotopes and the need to purify isotopes.


Each Isotope Has A Name


In order to specify a specific isotope among a set of isotopes present for a specific element, you use isotope or nuclide notation. This provides a method to tell the difference between carbon with a mass of 12.00 and carbon with a mass of 13.00.

To identify each isotope among other isotopes of the same element, Figure 3 shows isotope notation.


isotopic symbol notation
Figure 3: Isotope symbol used to denote each unique isotope.


X denotes any possible element. There is no X on the periodic table.


Z means “charge” or the atomic number. As you know, the atomic number gives you the number of protons. 


The superscript A stands for mass number; the sum of the protons and neutrons.

There are two major isotopes of chlorine, 3517Cl and 3717Cl. Notice the mass number is always a whole number. From the isotopic symbol you see for instance 35Cl has 17 protons and 18 neutrons. You find the number of neutrons when you subtract the atomic number from the mass number.


Notice also, you can specify 35Cl without the atomic number, because if you write Cl the only possible atomic number is 17. You should get used to the idea the atomic number and atomic symbol are the same thing. One automatically gives you the other.