TUTORIAL

 

What is an isotope?

Elements are defined by the number of protons (Z) in their nucleus. The mass number  (A) of an element is equal to the sum of both protons and neutrons (N) in the nucleus, or

 A = N + Z

A single element can have two or more mass numbers due to differences in the number of neutrons that can occur in the nucleus.  These different forms of a single element are called isotopes. While protons have a positive charge, neutrons have no charge, so the number of neutrons does not affect the charge of a molecule. Some isotopes are stable, while others are radioactive and release particles and energy to decay into a more stable form.

Elements usually have a common isotope that is the form most often found in nature. Because carbon, oxygen, and hydrogen are the elements that make up all organic matter, biologists are often interested in the isotopes of these elements.  Each has common and rare forms.  For instance, 98.8% of carbon atoms contain 6 protons and 6 neutrons with a mass number of 12; the notation for this form is 12C. 1.1% of carbon has 6 protons and 7 neutrons, noted as 13C. Similarly, 99.98% of hydrogen is found as 1H, but two stable isotopes and one radioactive isotope are known. 99.6% of oxygen is 16O, in addition there are three stable isotopes and five radioactive isotopes. Nitrogen, an important plant nutrient, is also of interest to biologists. It is found as 99.6% 14N and 0.4% 15N.

How common are stable isotopes?
A brief listing of the stable isotopes and their abundances for the elements most commonly used in global change research would include:

Element
Isotope
Abundance (%)
Hydrogen
1 H
99.985
 
2 H
0.015
Carbon
12 C
98.89
 
13 C
1.11
Nitrogen
14 N
99.63
 
15 N
0.37
Oxygen
16 O
99.759
 
17 O
0.037
 
18 O
0.204
Sulfur
32 S
95.00
 
33 S
0.76
 
34 S
4.22
 
36 S
0.014
Strontium
84 Sr
0.56
 
86 Sr
9.86
 
87 Sr
7.02
 
88 Sr
82.56

Isotopes influence the physical and chemical properties of matter. For instance, 12CO2 will behave differently than 13CO2 during certain processes and chemical reactions. Light isotopes form weaker chemical bonds than heavy isotopes, so light isotopes are somewhat more chemically reactive. Molecules containing light isotopes also evaporate and diffuse more quickly than their heavier counterparts. Therefore an evaporating liquid will contain more of the light isotope in the gas phase, and more of the heavy isotope in the liquid phase.

Isotopic fractionation occurs when isotopes are partitioned differently between two phases or two substances.  If we understand the processes underlying fractionation, we can use isotopes to determine how plants, animals, and whole ecosystems function. We can gain information from isotopes that naturally occur in the environment in studies of natural abundance, or we can artificially apply isotopes in high concentrations to use them as tracers.
 

 

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