atomic weight
C1Technical/Scientific
Definition
Meaning
The average mass of atoms of an element, measured in atomic mass units (amu), taking into account the relative abundances of its isotopes.
A dimensionless physical quantity representing the relative average mass of an element's atoms compared to one-twelfth the mass of a carbon-12 atom; historically used before the adoption of 'standard atomic weight' and 'relative atomic mass'.
Linguistics
Semantic Notes
The term 'atomic weight' is somewhat archaic in precise scientific contexts, having been largely replaced by 'relative atomic mass' or 'standard atomic weight', though it remains widely understood and used in educational and many applied contexts. It is a weighted average, not a fixed property of individual atoms.
Dialectal Variation
British vs American Usage
Differences
No significant differences in meaning or usage. Both regions use the term interchangeably with 'relative atomic mass' in educational settings.
Connotations
Slightly more old-fashioned connotation in both varieties, associated with mid-20th century textbooks. The modern IUPAC term 'standard atomic weight' is preferred in formal publications.
Frequency
Equally low frequency in general language, but standard in high school and undergraduate chemistry education in both the UK and US.
Vocabulary
Collocations
Grammar
Valency Patterns
The atomic weight of [ELEMENT] is [NUMBER].[ELEMENT] has an atomic weight of [NUMBER].to calculate/determine/find the atomic weightVocabulary
Synonyms
Strong
Neutral
Weak
Usage
Context Usage
Business
Virtually never used.
Academic
Used in chemistry, physics, and materials science textbooks and introductory courses.
Everyday
Rarely used outside educational or popular science contexts.
Technical
Core term in analytical chemistry, nuclear science, and stoichiometry calculations.
Examples
By Part of Speech
adjective
British English
- The atomic-weight value listed in the data sheet is crucial for the experiment.
American English
- We need the atomic-weight data from the latest IUPAC report.
Examples
By CEFR Level
- The atomic weight of oxygen is about 16.
- To find the molecular mass, you must first know the atomic weight of each element present.
- The atomic weight of chlorine is 35.45 because it is a mixture of isotopes.
- The precision of the analytical result depends on using the most recent standard atomic weight for the element.
- Historically, Dalton's determination of atomic weights was a foundational step in modern chemistry.
Learning
Memory Aids
Mnemonic
Think of a weighted average on a scale: the atomic WEIGHT is the average mass of all an element's atoms, weighted by how common each type (isotope) is.
Conceptual Metaphor
WEIGHT AS IMPORTANCE/VALUE (The 'weight' given to different isotopes determines the final average).
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Do not confuse with 'атомная масса' (atomic mass), which can refer to the mass of a single atom. 'Atomic weight' is specifically the average.
- The word 'weight' might misleadingly suggest a force (like вес), but here it means 'mass'.
Common Mistakes
- Using 'atomic weight' to refer to the mass of a single, specific atom.
- Confusing it with 'atomic number' (number of protons).
- Treating it as a constant for all samples of an element (it can vary slightly based on isotopic composition).
Practice
Quiz
What does 'atomic weight' specifically represent?
FAQ
Frequently Asked Questions
In casual usage, they are often used interchangeably. However, strictly speaking, 'atomic mass' can refer to the mass of a single atom (usually of a specific isotope), while 'atomic weight' is the weighted average mass of all naturally occurring isotopes of an element.
Because chlorine has two stable isotopes, chlorine-35 and chlorine-37. The atomic weight (35.45) is the average mass, weighted by the natural abundance of each isotope.
No. Atomic weight is a dimensionless relative measure. It is the ratio of the average mass per atom of the element to 1/12 of the mass of a carbon-12 atom. The unit is the atomic mass unit (amu or u), but the numerical value is often stated without a unit.
The International Union of Pure and Applied Chemistry (IUPAC) now recommends the terms 'relative atomic mass' (for a specific sample) and 'standard atomic weight' (for a best-known value). However, 'atomic weight' remains deeply entrenched in educational materials.