Chemistry / Unit 2 Chem 0249 short lesson, about 20 seconds
Elements Without a Standard Atomic Weight
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The idea
Elements whose isotopes are all radioactive have no natural abundance to average, so tables list their most stable isotope's mass number instead.
In practice
Technetium, promethium, and most elements from polonium onward show this mass number in brackets, not a weighted average.
Where people slip
A bracketed number is not an atomic weight — it is one isotope's mass number, not a mass in grams.
Your place on the trail
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Unlocks
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