noun

atomic mass

uh-TOM-ik mass

Atomic mass (noun) means the mass of a single atom, counted mainly from its protons and neutrons and usually given in atomic mass units, where one unit is a twelfth of the mass of a carbon-12 atom. Example: “The atomic mass of carbon-12 is exactly 12 atomic mass units by definition.”

noun

The mass of a single atom, counted mainly from its protons and neutrons and usually given in atomic mass units, where one unit is a twelfth of the mass of a carbon-12 atom.

"The atomic mass of carbon-12 is exactly 12 atomic mass units by definition."

"Because neutrons are added, each isotope of an element has a different atomic mass."

How to use atomic mass

Learner’s notes

In plain EnglishHow heavy one atom is, based mostly on its protons and neutrons.

When to use it

Chemistry and physics. The value printed under each element on the periodic table is strictly the relative atomic mass, an average across the isotopes found in nature, which is why it is rarely a whole number.

Memory tip

Electrons are so light that they barely affect the total, so atomic mass tracks the protons and neutrons almost exactly.

Easily confused with

mass number atomic number → molar mass atomic weight

Common pairings

atomic mass unit average atomic mass relative atomic mass atomic mass number

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Fill the gap

Can you complete this real example?

The _____ of carbon-12 is exactly 12 atomic mass units by definition.

Synonyms

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atomic weight relative atomic mass

Frequently asked questions

What is the difference between atomic mass and atomic number?

The atomic number is a count of protons and decides which element the atom is. The atomic mass is a measure of how heavy the atom is, and it depends on protons and neutrons together, so two atoms of the same element can share an atomic number but differ in atomic mass.

What is the difference between atomic mass and mass number?

The mass number is a whole number, the simple count of protons plus neutrons in one particular atom. Atomic mass is a measured quantity that comes very close to that count but is not exact, because binding energy and the tiny mass of electrons shift it slightly.

What is the meaning of atomic mass?

"atomic mass" (noun) means the mass of a single atom, counted mainly from its protons and neutrons and usually given in atomic mass units, where one unit is a twelfth of the mass of a carbon-12 atom.

What is another word for atomic mass?

Other words for "atomic mass" include atomic weight, relative atomic mass.