Atomic Weight
Definition and meaning of Atomic Weight in chemistry.
Atomic weight is the average mass of an atom of an element, expressed in atomic mass units (amu), based on the weighted average of all naturally occurring isotopes. It is a dimensionless quantity listed on the periodic table for every element.
In more detail
Unlike mass number, which refers to a specific isotope, atomic weight incorporates the relative abundance of each isotope found in nature. For example, chlorine exists as chlorine-35 (75.77% abundance) and chlorine-37 (24.23% abundance), yielding an atomic weight of 35.45 amu. Atomic weights are essential for stoichiometric calculations, molar mass determinations, and predicting chemical reactivity. The International Union of Pure and Applied Chemistry (IUPAC) maintains standardized atomic weight values for all elements.
Key facts
| Field | General Chemistry |
|---|---|
| Unit | atomic mass units (amu) |
| Also called | relative atomic mass |
| Differs from | mass number (integer value for a specific isotope) |
Carbon has an atomic weight of 12.01 amu because roughly 1.07% of natural carbon is carbon-13 (mass 13 amu), mixed with the predominant carbon-12 isotope (mass 12 amu).
Frequently asked questions
Why is atomic weight usually not a whole number?
Because elements contain multiple naturally occurring isotopes with different mass numbers. Atomic weight is the weighted average of these isotopes based on their relative abundances in nature.
How is atomic weight used in chemistry?
Chemists use atomic weights to calculate molar masses of compounds, balance chemical equations, and perform stoichiometric calculations for chemical reactions.