它是根据原子序数的增加来排列的，原子序数是一种元素单个原子的质子数。

It is arranged according to increasing atomic number, which is the number of protons in each atom of an element.

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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.

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我们从质子数开始。

And we began with the proton number.

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So carbon 12. We know that it has the proton number, by definition, is 6. And the neutron number, 6 from 12 is 6. So it has 6 protons and 6 neutrons.

所以碳12，我们知道它有质子数，根据定义，那就是6，而电子数，12减6等于6，所以它有6个中子。

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And it is equal to the sum of the number of protons plus the number of neutrons.

它等于质子数,加上中子数。

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And note that as Z increases, as the proton number increases the radius decreases for a given n number.

并注意到当Z不断增加，对于一个给定的n，即当质子数增加的时候,半径的n值就减小了。

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