noun

atomic mass unit

uh-TOM-ik MASS YOO-nit

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Atomic mass unit (noun) means a standard unit for stating the masses of atoms and molecules, defined as exactly one twelfth of the mass of a single carbon-12 atom, which is about 1.66 x 10^-27 kilograms. It is symbolised u or amu and is also called the dalton. Example: “A carbon-12 atom has a mass of exactly 12 atomic mass units.”

noun

technical A standard unit for stating the masses of atoms and molecules, defined as exactly one twelfth of the mass of a single carbon-12 atom, which is about 1.66 x 10^-27 kilograms. It is symbolised u or amu and is also called the dalton.

"A carbon-12 atom has a mass of exactly 12 atomic mass units."

"Express the molecular mass in atomic mass units rather than grams."

How to use atomic mass unit

Learner’s notes

In plain EnglishA tiny unit used to weigh atoms, based on the carbon-12 atom.

When to use it

Technical. Chemists and physicists increasingly prefer dalton (Da), especially in biochemistry.

Memory tip

One atomic mass unit is numerically the same as one gram per mole, which is why an element's atomic mass in u matches its molar mass in g/mol.

Trace the full origin ↓

Easily confused with

dalton → atomic mass → mass number molar mass

Common pairings

expressed in atomic mass units one atomic mass unit unified atomic mass unit atomic mass unit to kg

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Gen Z slang rizz Charm — the ability to attract a romantic interest.

B2 · Intermediate empathy The capacity to share another person’s feelings.

C1 · Advanced nonchalant Casually calm and relaxed; unconcerned.

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Etymology nostalgia Greek nostos “return home” + algos “pain”.

C2 · Advanced serendipity Fortunate discoveries made entirely by accident.

Gen Z slang rizz Charm — the ability to attract a romantic interest.

B2 · Intermediate empathy The capacity to share another person’s feelings.

C1 · Advanced nonchalant Casually calm and relaxed; unconcerned.

Internet slang delulu Delusional — believing something against the odds.

Etymology nostalgia Greek nostos “return home” + algos “pain”.

Test yourself on “atomic mass unit” A quick quiz — meaning, synonyms & usage →

Fill the gap

Can you complete this real example?

A carbon-12 atom has a mass of exactly 12 _____.

Etymology

A descriptive compound: atomic, from Greek atomos 'indivisible', plus mass unit.

Origin: English

Synonyms

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dalton unified atomic mass unit amu u

Related words

uranium Dalton

Rhymes for atomic mass unit

unit

See all rhymes for atomic mass unit →

Frequently asked questions

What is the difference between atomic mass unit and atomic mass?

The atomic mass unit is the unit of measurement; atomic mass is the quantity being measured. So an atom of oxygen-16 has an atomic mass of about 16, expressed in atomic mass units.

Is a dalton the same as an atomic mass unit?

Yes. The dalton (Da) and the unified atomic mass unit (u) are identical in value. Dalton is the name preferred by the international standards bodies and is standard in biochemistry, where protein masses are given in kilodaltons.

Why is carbon-12 used as the reference?

Carbon-12 is a stable, abundant isotope that can be measured very precisely in a mass spectrometer. Fixing its mass at exactly 12 gives a reference that produces whole-number-ish masses for most isotopes and avoids the inconsistencies of the older oxygen-based scales.

What is the meaning of atomic mass unit?

"atomic mass unit" (noun) means a standard unit for stating the masses of atoms and molecules, defined as exactly one twelfth of the mass of a single carbon-12 atom, which is about 1.66 x 10^-27 kilograms. It is symbolised u or amu and is also called the dalton.

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