The most common isotope of oxygen, for example, has an atomic weight of 16, but there are also rarer isotopes with weights of 17 and 18, caused by additional neutrons in their nuclei.

• 例如很常见的同位素氧，原子重量是16，但是还有更为稀少的重量为17和18的同位素，这是因为它们的原子核中有额外的中子。