138 中子数
Neutron Number
（https://www.nuclear-power.com/）
中子数
The element’s nuclear properties (atomic mass, nuclear cross-sections) are determined by the number of protons and the number of neutrons (neutron number). For example, actinides with odd neutron numbers are usually fissile (fissionable with slow neutrons), while actinides with even neutron numbers are not fissile (but fissionable with fast neutrons). Due to the Pauli exclusion principle, heavy nuclei with an even number of protons and an even number of neutrons are very stable thanks to the occurrence of ‘paired spin’. On the other hand, nuclei with an odd number of protons and neutrons are mostly unstable.
元素的核属性（原子质量，核截面）是由质子数和中子数量（中子数）决定的。例如，具有奇数中子数的锕系元素通常是可裂变的(可与慢中子裂变)，而具有偶数中子数的锕系元素则不可裂变（但可与快中子裂变）。由于泡利不相容原理，具有偶数个质子和偶数个中子的重核由于“成对自旋”的出现而非常稳定。另一方面，质子和中子数量为奇数的原子核大多是不稳定的。
Hydrogen (H), for example, consists of one electron and one proton. The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The total number of nucleons, protons, and neutrons in a nucleus are equal to Z N = A, where A is called the mass number. The various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Each nuclide is denoted by the element’s chemical symbol (this specifies Z) with the atomic mass number as superscript.
例如，氢（H）由一个电子和一个质子组成。原子核中的中子数量称为中子数，用符号N表示。原子核中核子、质子和中子的总数等于Z N = A，其中A称为质量数。原子核中含有特定数量的质子和中子的各种原子称为核素。每个核素都由元素的化学符号表示（这指定了Z），上标为原子质量数。
Thus the symbol 1H refers to the nuclide of hydrogen with a single proton as a nucleus. 2H is the hydrogen nuclide with a neutron and a proton in the nucleus (2H is also called deuterium or heavy hydrogen). Atoms such as 1H, 2H, whose nuclei contain the same number of protons but the different number of neutrons (different A), are known as isotopes. Hydrogen has identical chemical properties as deuterium but has completely different nuclear properties.
因此，符号1H指的是原子核只有一个质子的氢核素。2H是原子核中有一个中子和一个质子（2H也被称为氘或重氢）的氢核素。原子核中含有相同数量的质子但不同数量的中子（不同的A）的原子，如1H、2H，被称为同位素。氢与氘具有相同的化学性质，但具有完全不同的核性质。
（待续）
