Atomic Mass Unit (u) - Unit Information & Conversion
What is a Atomic Mass Unit?
Definition
The Atomic Mass Unit (symbol: u), also known as the Dalton (symbol: Da), is a standard unit of mass used to express the masses of atoms and molecules. It is defined as exactly one-twelfth (1/12) of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. This definition provides a convenient scale for comparing the masses of different isotopes and molecules.
History
The concept of a relative atomic mass scale emerged in the early 19th century with John Dalton. Initially, hydrogen was used as the standard. Later, oxygen became the standard (first as natural oxygen, then oxygen-16). In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the current standard based on carbon-12 to resolve discrepancies between the physics and chemistry scales that used oxygen-16 and natural oxygen, respectively. The name "Dalton" (Da) was proposed later and is now widely accepted, especially in biochemistry and molecular biology.
Common Uses
- Chemistry: Expressing atomic weights of elements and molecular weights of compounds. Essential for stoichiometry calculations.
- Physics: Measuring the masses of subatomic particles, nuclei, and atoms with high precision. Used in mass spectrometry.
- Biochemistry & Molecular Biology: Expressing the masses of large molecules like proteins and nucleic acids, often using the kilodalton (kDa) or megadalton (MDa).
Unit FAQs
What is the value of 1 u (or Da) in kilograms?
1 atomic mass unit (u) is approximately equal to 1.660 539 066 60(50) × 10⁻²⁷ kilograms (kg). The numbers in parentheses represent the uncertainty in the last digits.
Is the atomic mass unit (amu) the same as the Dalton (Da)?
Yes, the unified atomic mass unit (u) and the Dalton (Da) refer to the same unit of mass, defined based on carbon-12. While 'amu' might sometimes refer to older, slightly different definitions based on oxygen, modern usage treats 'u' and 'Da' as synonymous. The Dalton (Da) is often preferred in biological sciences.
Why was Carbon-12 chosen as the standard for atomic mass?
Carbon-12 was chosen as the reference standard in 1961 for several reasons:
- Relative Abundance: Carbon-12 is a reasonably abundant isotope (about 98.9% of natural carbon).
- Stability: It is a stable nuclide (not radioactive).
- Convenience: Its mass is close to an integer value (12), making it a convenient reference point.
- Resolution: It resolved the discrepancy between the older physics (based on oxygen-16) and chemistry (based on natural oxygen) mass scales.
Atomic Mass Unit Conversion Formulas
To Kilogram:
To Gram:
To Milligram:
To Pound:
To Ounce:
To Stone:
To Ton (metric):
To Ton (US):
To Ton (UK):
To Microgram:
To Carat:
To Slug:
To Troy Ounce:
To Pennyweight:
To Grain:
To Dram:
To Quintal:
To Pavan (India):
To Kati (India):
To Masha (India):
To Dina (India):
To Pras (India):
To Lota (India):
Convert Atomic Mass Unit
Need to convert Atomic Mass Unit to other weight units? Use our conversion tool.