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Atomic Weight

Ar = weighted average of isotope masses
Quick Reference
Formula / NotationAr = weighted average of isotope masses
Also Known AsRelative atomic mass; Ar; standard atomic weight; atomic mass; average atomic mass

What is Atomic Weight?

Atomic weight (also called relative atomic mass, Ar) is the weighted average mass of all naturally occurring isotopes of an element, relative to 1/12 the mass of carbon-12. It takes into account the natural abundance of each isotope. Atomic weight is dimensionless (or expressed in g/mol as molar mass) and appears on the periodic table.

Formula & Notation

Ar = weighted average of isotope masses

Other Names / Synonyms: Relative atomic mass; Ar; standard atomic weight; atomic mass; average atomic mass

Properties & Characteristics

Dimensionless ratio (relative to ¹²C/12); differs from mass number (integer); accounts for natural isotope distribution; listed on periodic table; approximately equals mass number of most abundant isotope; used in stoichiometric calculations; IUPAC provides official values with uncertainty ranges

Uses & Applications

Stoichiometric calculations; molar mass determination; isotope ratio analysis (mass spectrometry); nuclear physics; geochronology and radiometric dating; quality control in chemical manufacturing

Safety Information

No hazard — a chemical property.

Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.

Key Facts

Term Atomic Weight
Formula Ar = weighted average of isotope masses
Synonyms Relative atomic mass; Ar; standard atomic weight; atomic mass; average atomic mass

Frequently Asked Questions

Atomic weight (also called relative atomic mass, Ar) is the weighted average mass of all naturally occurring isotopes of an element, relative to 1/12 the mass of carbon-12. It takes into account the natural abundance of each isotope. Atomic weight is dimensionless (or expressed in g/mol as molar mass) and appears on the periodic table.

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