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Relative Atomic Mass vs. Atomic Mass: What's the Difference?

Edited by Janet White || By Harlon Moss || Updated on November 13, 2023
Relative atomic mass is the weighted average mass of an atom compared to one-twelfth the mass of a carbon-12 atom, while atomic mass is the mass of a specific isotope.

Key Differences

Relative atomic mass represents the average mass of the isotopes of an element, taking into account their abundance. It provides a mean value for the atomic masses of all isotopes of a particular element. Atomic mass, on the other hand, refers specifically to the mass of a single isotope of an element. It doesn't account for isotopic abundances.
Both relative atomic mass and atomic mass are essential concepts in the field of chemistry, especially in stoichiometry, where they help in determining the amounts of substances involved in reactions. While relative atomic mass gives a general idea about the average atomic mass of an element as it exists in nature, atomic mass offers specific information about a particular isotope.
It's vital to understand that while relative atomic mass considers the various isotopes and their natural abundances, atomic mass is a more precise measure. The atomic mass of an isotope can be determined through techniques like mass spectrometry. Relative atomic mass, meanwhile, is a calculated value based on the atomic masses of the different isotopes and their natural abundance.
In many contexts, especially for elements with one dominant isotope, the relative atomic mass and atomic mass can seem very close in value. However, for elements with multiple stable isotopes present in significant amounts, the distinction between relative atomic mass and atomic mass becomes crucial.

Comparison Chart


Weighted average of isotopic masses.
Mass of a specific isotope.


Abundance of isotopes.
Specific isotope.


General average for an element.
Precise value for an isotope.

Measurement Technique

Calculated based on isotopic masses & abundances.
Determined using mass spectrometry.


Stoichiometry, general chemistry.
Nuclear physics, isotope-specific work.

Relative Atomic Mass and Atomic Mass Definitions

Relative Atomic Mass

Represents the "average" atomic mass.
Since oxygen has three isotopes, its relative atomic mass is an average of the three.

Atomic Mass

Exact weight of an atom's nucleus.
The atomic mass of carbon-12 is exactly 12 atomic mass units.

Relative Atomic Mass

Value found on the periodic table.
The periodic table lists the relative atomic mass for each element.

Atomic Mass

Mass of a specific isotope.
The atomic mass of uranium-238 is 238 u.

Relative Atomic Mass

Weighted mean of atomic masses.
Carbon's relative atomic mass is close to 12 due to the dominance of the carbon-12 isotope.

Atomic Mass

Value specific to an isotope.
Deuterium's atomic mass is different from protium, even though both are isotopes of hydrogen.

Relative Atomic Mass

Average mass of isotopes based on abundance.
The relative atomic mass of chlorine considers both Cl-35 and Cl-37 isotopes.

Atomic Mass

Not influenced by isotopic abundance.
The atomic mass of carbon-13 remains constant regardless of its abundance in nature.

Relative Atomic Mass

Comparative measure against carbon-12.
Using carbon-12 as a standard, we determine the relative atomic mass of other elements.

Atomic Mass

Determined using mass spectrometry.
Scientists use mass spectrometry to find the atomic mass of rare isotopes.


Is atomic mass a fixed value for an element?

No, atomic mass varies among isotopes of the same element.

How is atomic mass different from relative atomic mass?

Atomic mass is the mass of a specific isotope, while relative atomic mass is an average for an element.

Why do some elements have close values for both relative atomic mass and atomic mass?

For elements with a dominant isotope, the relative atomic mass and atomic mass can be very similar.

Can relative atomic mass be a non-integer?

Yes, because it's an average of isotope masses and their abundances.

How is relative atomic mass calculated?

It's calculated based on the atomic masses of its isotopes and their natural abundance.

What provides the most specific mass information about an element?

The atomic mass of specific isotopes provides the most precise information.

Can atomic mass change?

Yes, when an atom undergoes nuclear reactions, its atomic mass can change.

How does atomic mass contribute to nuclear stability?

The atomic mass, particularly the neutron-to-proton ratio, can influence the stability of an atomic nucleus.

Which value is commonly found on the periodic table?

The periodic table usually lists the relative atomic mass for each element.

Which value is more relevant in nuclear physics?

Atomic mass, as it pertains to specific isotopes, is more relevant in nuclear physics.

Are both values measured in atomic mass units (u or amu)?

Yes, both relative atomic mass and atomic mass are typically expressed in atomic mass units.

What does relative atomic mass represent?

Relative atomic mass represents the weighted average mass of isotopes based on their abundance.

Why is carbon-12 often mentioned with relative atomic mass?

Carbon-12 is used as a standard reference for defining atomic mass units.

How does isotopic abundance affect relative atomic mass?

The relative atomic mass is a weighted average, so isotopic abundance greatly influences its value.

Can isotopes of an element have significantly different atomic masses?

Yes, isotopes have varying numbers of neutrons, leading to different atomic masses.

Which value becomes significant for elements with multiple stable isotopes?

Relative atomic mass is particularly important for elements with multiple stable isotopes.

Which value would be used to determine the amount of substance in chemical reactions?

Relative atomic mass is commonly used in stoichiometry and chemical reactions.

Are the terms "atomic weight" and "relative atomic mass" interchangeable?

Yes, "atomic weight" is often used synonymously with "relative atomic mass."

Does the atomic mass of an isotope consider electrons?

While electrons are part of an atom, their mass is negligible compared to protons and neutrons, so they have a minimal impact on atomic mass.

Why is understanding both values essential for chemists?

Understanding both provides a comprehensive view of an element's isotopic composition and its behavior in reactions.
About Author
Written by
Harlon Moss
Harlon is a seasoned quality moderator and accomplished content writer for Difference Wiki. An alumnus of the prestigious University of California, he earned his degree in Computer Science. Leveraging his academic background, Harlon brings a meticulous and informed perspective to his work, ensuring content accuracy and excellence.
Edited by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.

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