给定元素的所有同位素都含有相同的质子数，但它们的中子数不同，因此它们的原子质量也不同。
All the isotopes of a given element have the same number of protons, but differ in their number of neutrons, and hence in their atomic mass.
由于质子与中子的质量大约相等(并且电子几乎没有质量)，氦原子的质量大约应是氢原子质量的四倍。
Because protons and neutrons have roughly the same mass (and electrons have almost no mass), the helium atom should have about four times the mass of the hydrogen atom.
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