Difference Between Atomic Mass and Average Atomic Mass

The key difference between atomic mass and average atomic mass is that the atomic mass is the mass of an atom, whereas the average atomic mass is the mass of an atom of a particular chemical element calculated by considering isotopes of that element.

We often use the terms atomic mass and average atomic mass interchangeably; however, they are two different terms.

1. Overview and Key Difference
2. What is Atomic Mass
3. What is Average Atomic Mass
4. Side by Side Comparison – Atomic Mass vs Average Atomic Mass in Tabular Form
5. Summary

What is Atomic Mass?

Atomic mass is the total mass of nucleons present in the nucleus of an atom. A nucleon is either a p7p or a neutron. Hence, atomic mass is the total mass of the protons and neutrons present in the nucleus. Although electrons are also present in atoms, the mass of electrons is not used in the calculations because electrons are so small and have a negligible mass when compared to protons and neutrons.

Unlike in relative atomic mass, here we calculate the mass of each and every atom without calculating any average value. Therefore, we get different values for atomic masses of different isotopes. That is because the number of nucleons present in isotopes of the same element is different from each other.

Let us consider an example:

The atomic mass of hydrogen = 2

So, the atomic mass of hydrogen-2 (Deuterium) isotope is calculated as follows.

The number of protons in the nucleus = 1
The number of neutrons in the nucleus = 1
Therefore, the atomic mass of hydrogen = (1 amu + 1 amu) = 2 amu

Here, tThe atomic mass is given by the unit amu (atomic mass units). One proton or a neutron has a mass of 1 amu.

What is Average Atomic Mass?

Average atomic mass is the mass of an atom of a particular chemical element calculated by considering the isotopes of that element. In this, the mass value depends on the natural abundance of a chemical element.

There are two steps to calculate the average atomic mass.

Multiply the atomic mass of each isotope from the natural abundance(taking the abundance as a percentage) separately.
Add the obtained values together to get the average atomic mass.

Let us consider an example:

There are two isotopes of carbon as carbon-12 and carbon-13. Their abundance is 98% and 2, respectively. Then we can determine the average atomic mass of carbon using a calculation. Here, we have to multiply the atomic masses of each isotope with the abundance value. Then, we need to take the abundance as a two decimal placed value, not as the percentage. Next, we can add the obtained values.

Carbon-12: 0.98*12=11.76

Carbon-13: 0.02*13=0.26

Then, average atomic mass of carbon is=11.76+0.26=12.02 g/mol.

What is the Difference Between Atomic Mass and Average Atomic Mass?

Although the term atomic mass and average atomic mass sound similar, they are two different terms. The key difference between atomic mass and average atomic mass is that the atomic mass is the mass of an atom, whereas the average atomic mass is the mass of an atom of a particular chemical element calculated by considering isotopes of that element. The term atomic mass refers to the mass of a single atom while the term average atomic mass refers to a mass of a chemical element.

Summary – Atomic Mass vs Average Atomic Mass

Atomic mass refers to the mass of a single atom, but average atomic mass refers to the average mass of an atom of a particular chemical element. Therefore, the key difference between atomic mass and average atomic mass is that the atomic mass is the mass of an atom, whereas the average atomic mass is the mass of an atom of a particular chemical element calculated by considering isotopes of that element.

Reference:

Helmenstine, Anne Marie. “How to Calculate Atomic Mass.” ThoughtCo, Aug. 21, 2019, Available here.
Helmenstine, Anne Marie. “How to Calculate Atomic Mass.” ThoughtCo, Aug. 21, 2019, Available here.

Image Courtesy:

1. “Stylised atom with three Bohr model orbits and stylised nucleus” By SVG by Indolences. Recoloring and ironing out some glitches done by Rainer Klute. – Own work based on: of Image: Stylised Lithium Atom.png by Halfdan (CC BY-SA 3.0) via Commons Wikimedia