How Many Protons, Neutrons and Electrons? | 如何计算质子、中子和电子数？

📚 How Many Protons, Neutrons and Electrons? | 如何计算质子、中子和电子数？

Counting subatomic particles is one of the most basic but frequently examined skills in Cambridge A Level Chemistry. You must be able to use atomic number (Z), mass number (A) and ionic charge to state the number of protons, neutrons and electrons in atoms, ions and isotopes.

计算亚原子粒子数是剑桥 A Level 化学中最基础但高频考查的技能之一。你必须能够利用原子序数（Z）、质量数（A）和离子电荷，说出原子、离子和同位素中的质子数、中子数和电子数。

1. Nuclear Symbols and the Meaning of Z and A | 核素符号以及 Z 与 A 的含义

A nuclide is written with the mass number A as a superscript on the left of the element symbol, and the atomic number Z as a subscript on the left. This is often called the AZX notation or nuclide notation.

核素写作元素符号左侧上标质量数 A、下标原子序数 Z。这种写法通常称为 AZX 符号或核素符号。

• Z = atomic number = number of protons in the nucleus.
• Z = 原子序数 = 原子核中的质子数。
• A = mass number = total number of protons + neutrons.
• A = 质量数 = 质子数 + 中子数。

For example, ¹²₆C has Z = 6 and A = 12. The lower number is always the atomic number, and the upper number is always the mass number.

例如，¹²₆C 的 Z = 6，A = 12。下方数字始终是原子序数，上方数字始终是质量数。

2. Protons: Always from the Atomic Number | 质子：始终由原子序数确定

The number of protons is equal to the atomic number Z. This value is fixed for a given element and does not change when the atom forms an ion or when the isotope changes.

质子数等于原子序数 Z。对于给定元素，这个数值是固定的，不会因为形成离子或同位素改变而改变。

If you know the element is chlorine, the proton number must be 17 because chlorine has Z = 17. Changing the number of protons would change the identity of the element.

只要知道该元素是氯，质子数就一定是 17，因为氯的 Z = 17。改变质子数会改变元素的种类。

protons = Z

This is the simplest step, so always write down Z first before moving to neutrons and electrons.

这是最简单的一步，因此总是先写下 Z，再计算中子数和电子数。

3. Neutrons: Mass Number Minus Atomic Number | 中子：质量数减去原子序数

The mass number A counts all nucleons, which are the protons and neutrons in the nucleus. Since protons are counted by Z, the neutron number N is obtained by subtracting Z from A.

质量数 A 计算所有核子，即原子

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