一种元素中原子的平均重量，通常相对碳2的重量来表示，碳2的重量定为十二个原子量单位。
The average mass of an atom of an element, usually expressed relative to the mass of carbon 2, which is assigned 2 atomic mass units.
一种元素中原子的平均重量，通常相对碳12的重量来表示，碳12的重量定为十二个原子量单位。
The average mass of an atom of an element, usually expressed relative to the mass of carbon 12, which is assigned 12 atomic mass units.
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