Dictionary > Unified atomic mass unit

Unified atomic mass unit

noun, plural: unified atomic mass units
One twelfth the mass of a neutral atom of the most abundant isotope of carbon (carbon-12)
The unified atomic mass unit (u) refers to the 1/12 (0.0833) the mass of the carbon-12 atom. It is used to express the mass of atomic and subatomic particles.
It is also called dalton, named after John Dalton. John Dalton proposed in 1803 to use the mass of hydrogen atom as the natural unit for relative atomic mass. The hydrogen was replaced later on with oxygen. Wilhelm Ostwald suggested to base the relative atomic mass in terms of 1/16 mass of oxygen in 1903. Thus, formerly, the atomic mass unit is based on oxygen. Years later (in 1929), isotopic oxygen was discovered and as such it resulted in the divergence in relative atomic mass representation. The divergence could result in errors. To prevent this, carbon-12 was eventually used as the reference standard in 1961.
At this point in time, the atomic mass unit is based on carbon-12. In so doing, it was called unified atomic mass unit. Nevertheless, the term atomic mass unit without the unified is still being used today although it is no longer based on oxygen-16 but on carbon-12.

  • u
  • Da


  • dalton
  • atomic mass unit (AMU)

    You will also like...

    Thermographic image of face and neck
    Regulation of Organic Metabolism, Growth and Energy Balance

    The human body is capable of regulating growth and energy balance through various feedback mechanisms. Get to know the e..

    Lotic community of algal species
    Lotic Communities & Algae

    Lotic communities have conditions that are rather harsh for typical plants. Thus, the diversity of plant species in loti..

    Homeostatic Mechanisms and Cellular Communication
    Homeostatic Mechanisms and Cellular Communication

    Homeostasis is the relatively stable conditions of the internal environment that result from compensatory regulatory res..

    Biosecurity and Biocontrol
    Biosecurity and Biocontrol

    This lesson explores the impact of biosecurity threats, and why they need to be identified and managed. Examples to incl..

    Running Water Freshwater Communities

    This tutorial introduces flowing water communities, which bring new and dithering factors into the equation for possible..

    Early Earth
    The Origins of Life

    This tutorial digs into the past to investigate the origins of life. The section is split into geological periods in the..