Natural Abundance Definition

Natural abundance is the measure of the average amount of a given isotope naturally occurring on Earth. The abbreviation for natural abundance is NA. The atomic weight listed for each element on the periodic table is the natural abundance on Earth. Sometimes the value changes as scientists obtain more data about the isotope ratio of samples. The natural abundance of elements on the periodic table is not the same everywhere in the universe. The ratio of isotopes in the Sun or on Mars, for example, might be different.

Example

There are two natural isotopes of boron: ¹⁰B and ¹¹B. The natural abundance is 19.9% of ¹⁰B and 80.1% of ¹¹B. Put another way, if you took a 100-gram sample of boron from anywhere on the planet, you could expect 19.9 grams to consist of boron-10 and 80.1 grams to consist of boron-11.

Deviations

The natural abundance is a global mean, so if you sample an element at one location, you won't get exactly the average ratio of elements. Why is this so? Scientists believe the chemical composition of the solar system was isotopically homogeneous during its formation, but that deviations began to occur when fusion started in the Sun. Also, radioactive decay leads to differences in isotope ratios. This is because decay is a random process.

Sources

• Clayton, Robert N. (1978). "Isotopic anomalies in the early solar system". Annual Review of Nuclear and Particle Science. 28: 501–522.
• Lide, D. R., ed. (2002). CRC Handbook of Chemistry and Physics (83rd ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0483-0. 
• Zinner, Ernst (2003). "An isotopic view of the early solar system". Science. 300 (5617): 265–267.