万物简史 第198期:威力巨大的原子(12)

 　　Let us pause for a moment and consider the structure of the atom as we know it now. Every atom is made from three kinds of elementary particles: protons, which have a positive electrical charge; electrons, which have a negative electrical charge; and neutrons², which have no charge. Protons and neutrons are packed into the nucleus³, while electrons spin around outside. The number of protons is what gives an atom its chemical identity. An atom with one proton is an atom of hydrogen, one with two protons is helium, with three protons is lithium, and so on up the scale. Each time you add a proton you get a new element. (Because the number of protons in an atom is always balanced by an equal number of electrons, you will sometimes see it written that it is the number of electrons that defines an element; it comes to the same thing. The way it was explained to me is that protons give an atom its identity, electrons its personality.)

　　让我们稍停片刻，先来考虑一下现在我们所知道的原子结构。每个原子都由三种基本粒子组成：带正电荷的质子，带负电荷的电子，以及不带电荷的中子。质子和中子装在原子核里，而电子在外面绕着旋转。质子的数量决定一个原子的化学特性。有一个质子的原子是氢原子；有两个质子的原子是氦原子；有三个质子的原子是锂原子；如此往上增加。你每增加一个质子就得到一种新元素。（由于原子里的质子数量总是与同样数量的电子保持平衡，因此你有时候会发现有的书里以电子的数量来界定一种元素，结果完全一样。有人是这样向我解释的：质子决定一个原子的身份，电子决定一个原子的性情。）

　　原子结构

　　Neutrons don't influence an atom's identity, but they do add to its mass. The number of neutrons is generally about the same as the number of protons, but they can vary up and down slightly. Add a neutron¹ or two and you get an isotope⁴. The terms you hear in reference to dating techniques in archeology refer to isotopes—carbon-14, for instance, which is an atomof carbon with six protons and eight neutrons (the fourteen being the sum of the two).

　　中子不影响原子的身份，但却增加了它的质量。一般来说，中子数量与质子数量大致相等，但也可以稍稍多一点或少一点。增加或减少一两个中子，你就得到了同位素。考古学里就是用同位素来确定年代的——比如，碳－14是由6个质子和8个中子组成的碳原子（因为二者之和是14）。

1 neutron   

n.中子

参考例句：

• Neutron is neutral and slightly heavier than the proton.中子是中性的，比质子略重。
• Based on the neutron energy,the value of weighting factor was given.根据中子能量给出了相应的辐射权重因子的数值。

2 neutrons   

n.中子( neutron的名词复数 )

参考例句：

• The neutrons and protons form the core of the atom. 中子和质子构成了原子核。 来自《简明英汉词典》
• When an atom of U235 is split,several neutrons are set free. 一个铀235原子分裂时，释放出几个中子。 来自《简明英汉词典》

3 nucleus   

n.核，核心，原子核

参考例句：

• These young people formed the nucleus of the club.这些年轻人成了俱乐部的核心。
• These councils would form the nucleus of a future regime.这些委员会将成为一个未来政权的核心。

4 isotope   

n.同位素

参考例句：

• The isotope ratio is directly used for comparing oils or gases.同位素比率直接用于比较各种石油或天然气。
• How to apply a radio isotope?如何运用放射性同位素？