Beginner Topic 25 of 1102 Atomic Structure & Periodicity
Atomic Mass Unit
A mass unit equal to one twelfth of the mass of a carbon-12 atom, about 1.66 x 10^-27 kg.
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In-depth explanation of Atomic Mass Unit
The atomic mass unit, symbol u, is defined as one twelfth of the mass of a carbon-12 atom. Its value is about 1.6605 x 10^-27 kg, which equals 1 g/mol divided by the Avogadro constant. The unit lets chemists express atomic and molecular masses as convenient small numbers instead of tiny values in kilograms. One u corresponds to 1 g/mol at the molar scale, which links atomic masses to everyday grams. The unit is also called the dalton (Da), especially in biochemistry.
Going deeper: Atomic Mass Unit
Key ideas: One u (or Da) is exactly 1/12 of the mass of a 12C atom, equal to 1.6605 x 10^-27 kg or 931.5 MeV/c2. Use it to convert: mass of one atom in kg = mass in u x 1.6605 x 10^-27. Numerically the atomic mass in u equals molar mass in g per mol.
Common mistakes and connections: Students think 1 u equals the mass of a proton or hydrogen atom exactly; it is slightly less (proton 1.0073 u). It was previously defined with oxygen-16, an older convention. Because of the mass defect, the mass of a nucleus is less than the sum of its nucleons, linking to binding energy.
Visualization of Atomic Mass Unit
One atomic mass unit is 1/12 of a carbon-12 atom, so protons and neutrons each weigh about 1 u and C-12 is exactly 12 u. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A water molecule has a mass of about 18.02 u, so one mole of water has a mass of 18.02 g.
Frequently asked questions about Atomic Mass Unit
What is Atomic Mass Unit?
A mass unit equal to one twelfth of the mass of a carbon-12 atom, about 1.66 x 10^-27 kg.
Is Atomic Mass Unit a beginner, intermediate or advanced topic?
Atomic Mass Unit is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Atomic Mass Unit?
Study these earlier topics in Atomic Structure & Periodicity first: Isobars, Isotones, Relative Atomic Mass.
What comes after Atomic Mass Unit?
The next topics in Atomic Structure & Periodicity are Mole and Avogadro's Number.
Can I practise Atomic Mass Unit with quizzes?
Yes. The interactive lesson includes a self-check question on Atomic Mass Unit, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Atomic Mass Unit fits in Atomic Structure & Periodicity
Atomic Mass Unit is topic 25 of 115 in Atomic Structure & Periodicity.