Difference Between Percent Abundance and Relative Abundance

Key Difference – Percent Abundance vs Relative Abundance
 

Percent abundance and relative abundance are percentage values of chemical elements that represent their occurrence in the environment. The key difference between percent abundance and relative abundance is that percent abundance gives the abundance of isotopes whereas relative abundance gives the abundance of chemical elements. The percent abundance can be used to determine the average atomic mass of a certain chemical element. Relative abundance gives the occurrence of a certain chemical element in a given environment, i.e, on earth.

CONTENTS

1. Overview and Key Difference
2. What is Percent Abundance 
3. What is Relative Abundance
4. Similarities Between Percent Abundance and Relative Abundance
5. Side by Side Comparison – Percent Abundance vs Relative Abundance in Tabular Form
6. Summary

What is Percent Abundance?

Percent abundance is the percentage amount of all naturally occurring isotopes of an element. Isotopes are atoms of the same element that have identical atomic numbers but different mass numbers. This means isotopes are atoms having the same number of protons in the atomic nucleus, but different numbers of neutrons.

Isotopes of each element occur naturally in different ratios. The percent abundance of an isotope indicates the probability of finding that isotope in nature since elements can be found as a mixture of isotopes. The percent abundance can be used to find the atomic mass of the element. The atomic mass can be found using the following equation.

Average atomic mass = ∑ (mass of isotope x percent abundance of isotope)

Let us consider an example to understand this. The most stable, naturally occurring isotopes of chlorine are Cl-35 (mass = 34.969 and percent abundance = 75.53%) and Cl-37 (mass = 36.966 and percent abundance = 24.47%). Then,

The average mass of chlorine = ∑ (mass of isotope x percent abundance of isotope)

= ∑ (34.969 x {75.53/100}) + (36.966 x {24.47/100})

= 26.412 amu + 9.045 amu

= 35.46 amu.

What is Relative Abundance?

Relative abundance of an element is a measure of the occurrence of an element relative to all other elements in the environment. There are three ways to determine the relative abundance of an element:

1. Mass fraction
2. Mole fraction
3. Volume fraction

Volume fraction method is most common for gaseous elements in gas mixtures, i.e, the atmosphere of earth. However, most relative abundance expressions are mass fractions.

When considering the universe, the most abundant chemical elements are hydrogen and helium. When considering the earth, the most common element is iron whose mass percentage is 32.1%. Other elements are oxygen (32.1%), silicone (15.1%), magnesium (13.9%), sulfur (2.9%) and other elements are present in trace percentages.

What are the Similarities Between Percent Abundance and Relative Abundance?

• Both percent abundance and relative abundance are percentage values.
• Both percent abundance and relative abundance express the percentages of different chemical elements.

What is the Difference Between Percent Abundance and Relative Abundance?

Summary – Percent Abundance vs Relative Abundance

Percent abundance and relative abundance are two terms used to give the abundance of isotopes and chemical elements. The key difference between percent abundance and relative abundance is that percent abundance gives the abundance of isotopes whereas relative abundance gives the abundance of chemical elements.

Reference:

1. “Average Atomic Mass.” Average Atomic Mass, Physics@TutorVista.com, Available here.
2. “Abundance of the chemical elements.” Wikipedia, Wikimedia Foundation, 25 Feb. 2018, Available here.
3. Simoes, Christian. “Abundance of the chemical elements.” Astrono, Available here.

Image Courtesy:

1. “Isotope CNO” By Lucquessoy – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Elemental abundances” By Gordon B. Haxel, Sara Boore, and Susan Mayfield from USGS; vectorized by User:michbich – (Public Domain) via Commons Wikimedia