Isotopes, Atomic Weights and Abundances

An isotope is a variation of an element which has more or fewer neutrons while having the same number of protons. Remember, it is the number of protons which define the number of electrons and therefore define the element’s chemical properties. Some of an elements isotopes may be unstable and one of the excess neutrons can decay into a proton plus an energetic electron (Beta emission). Carbon 14 is a radioactive isotope of Carbon 12 and is used to determine the age of fossils.

Watch this video then draw the nucleus of Carbon 12 and the nucleus of Carbon 14.

Here are some practice problems to help you understand atomic number and atomic mass of isotopes.



Since several isotopes of an element may occur naturally in nature, we need to examine how the isotopes mix to give us the correct atomic weight.

Th first step is to understand “abundance”. Abundance is a number that tells us how common one particular isotope is.  Abundances are  usually expressed in percent or in parts per million or as a decimal.

The next thing to understand is that atomic weights are measured in AMU’s or Atomic Mass Units. 1 AMU is 1/12th the weight of a Carbon 12 nucleus. Carbon 12 has 6 protons and 6 neutrons.

Example:  The abundance of Europium  151 is 48.03% and the abundance of Europium 153 is 51.97%.

Step 1: convert % to decimals.
Eu 151:   48.03%  = 0.4803
Eu 153:   51.97% = 0.5197

Step 2:  Now multiply the atomic weight of each isotope times its abundance.
Eu 151:   151 AMU x  0.4803  = 72.53 AMU
Eu 153:   153 AMU x  0.5197 = 79.51 AMU

Step 3:  Add these numbers together to get the average atomic weight of Europium

72.53 AMU  + 79.51 AMU = 152.0 AMU

The following worksheet provides lots of practice for calculating atomic weights.


Now let’s talk about how Carbon dating is used. Watch the first 2 minutes of this video, then answer the question.

Assume Carbon 14 is created at a constant rate in the upper atmosphere. Knowing that the amount of Carbon 12 is constant while Carbon 14 decays, how long will it take for the amount of Carbon 14 in a dead animal such as a dinosaur to reduce to half its original amount.

Explain how Carbon 14 is used to determine the age of fossils.


How do we know isotopes actually exist ?

Elements can be separated into isotopes using a  mass spectrometer.

mass-spectrometer_3 mass_spectrometer_2The material to be analyzed is placed in a crucible inside the vacuum system. A beam of electrons heats the saple till it starts to vaporize.  The vaporized gas atoms are then ionized, accelerated through an electrostatic potential (Voltage). then separated using a magnetic field.

The number of atoms detected along each path is used to

mass-spectrometer-4determine the Isotopic Abundance of each species.