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Relative Isotopic Mass

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Relative isotopic mass is the mass of an isotope of an atom compared to 1/12th the mass of a carbon-12 atom.

Relative isotopic mass has no units because it is a ratio of the mass of an atom to the mass of a carbon-12 atom. A carbon-12 has a mass of exactly 12. The mass of an isotope is usually stated to one decimal place. In most cases, you can assume that the relative isotopic mass is the same as the mass number A.

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Specification: OCR Chemistry A (H032), AS-level, 2.1.1 Atomic structure and isotopes. Topic: Atoms, Ions and Compounds.

Key terms in this lesson

Isotopes
Atoms of the same element with the same number of protons but different numbers of neutrons.
Mass number
The number of protons and neutrons in the nucleus of an atom.

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