relative atomic mass
Definition
Noun 1. (Chemistry) The average mass of an atom of a chemical element, expressed in atomic mass units (amu or u). It is a dimensionless physical quantity calculated as the weighted average of the masses of all naturally occurring isotopes of an element, relative to one-twelfth the mass of an atom of carbon-12.
Usage
- This term is used in scientific contexts, primarily in chemistry and physics, to specify the mass of atoms for calculations and comparisons.
- It is often used interchangeably with "atomic weight" in many contexts, though "relative atomic mass" is the more precise modern term.
- It is typically followed by the element it describes.
Examples
- The relative atomic mass of hydrogen is approximately 1.008.
- To calculate the mass of a compound, you must sum the relative atomic masses of its constituent elements.
- The periodic table lists the relative atomic mass for each element.
Advanced Usage
- "Relative isotopic mass" is a related term referring to the mass of a specific isotope of an element, relative to the carbon-12 standard.
- The value is a weighted average, meaning it accounts for the abundance of each isotope in a naturally occurring sample. For example, chlorine's relative atomic mass of 35.45 reflects the mix of chlorine-35 and chlorine-37 isotopes.
Variants and Related Words
- Atomic Weight (n): A largely synonymous term, though historically it was defined differently. In modern use, it is often considered equivalent to relative atomic mass.
- Standard Atomic Weight (n): A more specific value published by IUPAC, representing the best knowledge of atomic weight variations in normal materials.
Synonyms
- Atomic weight (in modern, non-technical contexts)
Notes
- The value is dimensionless because it is a ratio (mass of an atom / (1/12 mass of carbon-12 atom)).
- It is a crucial value for stoichiometric calculations in chemistry, such as determining molar masses and balancing chemical equations.
Noun
-
(chemistry) the mass of an atom of a chemical element expressed in atomic mass units
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