A Level Chemistry OCR Practice Exam

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What does relative isotopic mass represent?

The mass of a molecule compared to carbon-12
The mass of an atom of a particular isotope compared to one-twelfth of the mass of carbon-12
The average mass of all isotopes of an element
The mass of an atom in a solid-state

The correct answer is: The mass of an atom of a particular isotope compared to one-twelfth of the mass of carbon-12

Relative isotopic mass specifically refers to the mass of an atom of a particular isotope, measured relative to one-twelfth of the mass of a carbon-12 atom. This standardization to carbon-12 is key in defining the mass scale used in chemistry, allowing for consistent and comparable measurements of atomic masses across all elements. Each isotope of an element has a different number of neutrons and therefore a different mass, and the relative isotopic mass highlights this difference. The choice of carbon-12 as a reference is universally accepted in chemistry, as it is a stable isotope and abundant in nature. This concept is crucial when discussing isotopes, their properties, and their applications in various scientific fields, including nuclear chemistry and radiocarbon dating. By this definition, the relative isotopic mass provides a precise means to compare isotopes of elements within a standardized framework.