Atomic weight is the average weight of a single atom of an element, measured using a complex system based on the weight of reference elements. Most elements have multiple isotopes, which affects the measurement. The International Union of Pure and Applied Chemistry publishes standard atomic weights.
Atomic weight is the weight of the atoms of an element. The science behind this measurement is extremely complex and has changed quite a bit since the early 1800s when the concept was first proposed. It is also important to distinguish between weight and mass: atomic weight, also called relative atomic mass, reflects the average weight of a single atom of an element, while atomic mass refers to the weight of a specific atom.
Simple textbooks and science courses often describe atomic weight as the number of protons and neutrons in an element added together. An element with two protons and two neutrons, for example, would be considered to have a weight of four in this system. Unfortunately, neutrons actually weigh more than protons, which makes this rough guideline a bit inaccurate.
Individual atoms are extremely small and quite difficult to weigh on their own. Early researchers came up with a system that involved determining the weight of one element relative to the weight of another. Several reference elements have been proposed, including hydrogen, with current measurements derived from carbon-12, a stable and abundant form of carbon. An atomic mass unit, the unit of measurement used to arrive at atomic weight, is 1/12 the weight of carbon-12.
There is another complexity to add to the equation, which is that most elements exist in the form of multiple isotopes, each of which has a slightly different mass. Relative atomic mass measurements actually reflect an average of measurements taken from samples of all known isotopes of an element. In the case of elements with only one or two known isotopes, the weight is often very close to a whole number, but in the case of other elements, the exact measurement can add several digits after the decimal point.
The International Union of Pure and Applied Chemistry regularly publishes lists of atomic weights used as standards in the scientific community. For quick calculations, especially in introductory basic science lessons, the old “protons plus neutrons” formula is sometimes used, but in advanced science it is important to use a more precise measurement. Particularly finicky scientists can actually take some time to determine the specific atomic mass of an element they’re working with, as small variations between isotopes can make huge differences in experiments.
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