Definition

1. Noun:
   • (Chemistry) The mass of an atom of a chemical element expressed in atomic mass units: It is a weighted average of the masses of all the naturally occurring isotopes of an element, relative to the mass of an atom of carbon-12, which is defined as exactly 12 atomic mass units.

Usage

• As a subject or object in a sentence: The term "atomic weight" is used to discuss the relative mass of an element's atoms in chemical calculations and discussions.
• Key Point: Although the term uses "weight," it is a measure of relative atomic mass and is dimensionless, but is expressed in atomic mass units (u or Da).

Examples

• Noun:
  • The atomic weight of hydrogen is approximately 1.008.
  • In the periodic table, elements are arranged in order of increasing atomic number, not atomic weight.
  • Scientists use the atomic weight to calculate molecular weights for compounds.

Advanced Usage

• "Standard atomic weight": Refers to the recommended value published by IUPAC (International Union of Pure and Applied Chemistry), which is often given as an interval to reflect natural isotopic variation.
  • The standard atomic weight of boron is listed as [10.806, 10.821].
• Distinction from "atomic mass": In modern terminology, "atomic weight" is often used synonymously with "relative atomic mass," while "atomic mass" can refer to the mass of a specific isotope.
  • The atomic mass of carbon-12 is exactly 12 u, while its atomic weight is 12.011.

Variants and Related Words

• Relative atomic mass (Aᵣ): The modern, more precise term for atomic weight.
• Atomic mass unit (amu or u): The standard unit of mass used for expressing atomic and molecular masses. One atomic mass unit is defined as one-twelfth of the mass of a carbon-12 atom.
• Isotopic mass: The mass of a specific isotope of an element.

Synonyms

• Relative atomic mass: The preferred term in modern scientific contexts.
• Mean atomic mass: Emphasizes it is an average.

Related Phrases

• To determine the atomic weight: A phrase describing the process of calculating or measuring this value.
  • Early chemists worked to determine the atomic weight of elements.
• Based on atomic weight: A phrase indicating a calculation or order uses this value.
  • Mendeleev's first periodic table was based on atomic weight.

Noun

1. (chemistry) the mass of an atom of a chemical element expressed in atomic mass units