Atomic Mass Unit (AMU): Definition, Standard & Conversion

Lesson Transcript
Instructor: Felicia Fullilove
We know all matter is made of atoms, but how is the mass of an atom expressed? Explore this lesson to learn more about atomic mass units (amus), carbon-12, and amu to kilogram conversion!

What Is an Atom?

How much do you weigh? No need to answer out loud, but whatever the answer, I am sure the amount was expressed in pounds or kilograms. What are you made of? You may answer organs, blood, or bones. The simplest answer, however, is atoms and molecules. An atom is the smallest constituent of a chemical element that has all the properties of an element. Atoms comprise every single piece of matter. From your sofa to the sandwich you ate yesterday, atoms in multiple combinations, or molecules, are all around us. But how is the mass of an atom expressed? In this lesson, we will learn more about atomic mass units.

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  • 0:02 What Is an Atom?
  • 0:42 Atomic Mass Unit (AMU)
  • 1:27 AMU Standard
  • 2:39 Converting Units
  • 5:24 Lesson Summary
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Atomic Mass Unit (AMU)

The periodic table of elements contains every atom known to mankind. Each unique atom has a unique atomic number and atomic mass. The atomic number is the number of protons in the atom's nucleus, while the atomic mass is the mass of the atom, which is the sum of the number of protons and neutrons. The atomic mass of an element is expressed in atomic mass units. Atomic mass units are described as a unit of measurement for atoms and molecules, just like the mass of a person may be expressed in pounds or kilograms. Hydrogen, for example, is the first element on the periodic table and has an atomic number of 1 and an atomic mass of 1.00794 amu, or atomic mass units.

AMU Standard

According to the International Union of Pure and Applied Chemistry (IUPAC), 1 atomic mass unit is defined as 1/12 the mass of carbon-12. Therefore, carbon-12 has a mass of 12 amu. Carbon-12 is the most abundant isotope of carbon. An isotope is two or more elements with the same number of protons (atomic number) but different number of neutrons. For example, carbon-12 has 6 protons, 6 electrons, and 6 neutrons and encompasses over 98% of carbon found in nature. On the other hand, carbon-13 has 6 protons, 6 electrons, and 7 neutrons; and carbon-14 has 6 protons, 6 electrons, and 8 neutrons.

Why is carbon the standard? Good question! Carbon is commonly combined with other elements; additionally, the carbon-12 isotope is highly abundant in nature. After many years of arguing, chemists and physicists agreed that carbon-12 would be the best unit of measurement for atomic masses. Previous standards included hydrogen and oxygen; however, scientists had difficulty using these elements as standards due to the distribution of their isotopes.

Converting Units

If atoms are weighed in atomic mass units, how do we get from atomic mass units to a more common measurement like kilograms? Conversion! According to the Royal Society of Chemistry, one atomic mass unit is equal to 1.66 x 10^-27 kg. Where did this number come from? While the history of atomic masses is quite complicated, the conversion factor from atomic mass units to kilograms is related to the mole and Avogadro's constant.

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