The SI unit for atomic mass is the kilogram (kg).
While the kilogram is the official SI unit, atomic mass is often expressed in daltons (Da), also known as atomic mass units (amu), because using kilograms directly results in extremely small and unwieldy numbers. A dalton is defined as 1/12 of the mass of a neutral carbon-12 atom in its ground state.
Here's a breakdown:
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SI Unit: The International System of Units (SI) specifies the kilogram (kg) as the base unit for mass, including atomic mass.
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Practical Usage: Due to the tiny masses of atoms, using kilograms directly is inconvenient. Therefore, the dalton (Da) is widely used in chemistry and biochemistry. 1 Da ≈ 1.66054 × 10⁻²⁷ kg.
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Dalton (Da) Definition: The dalton is a non-SI unit accepted for use with the SI. It's defined based on the mass of carbon-12.
| Unit | Symbol | Value in kg (approximate) |
|---|---|---|
| Kilogram | kg | 1 |
| Dalton (amu) | Da | 1.66054 × 10⁻²⁷ |
In summary, although the official SI unit for atomic mass is the kilogram, the dalton is the de facto unit due to its practicality in representing the mass of individual atoms and molecules.
