How Many Protons, Neutrons and Electrons? | 质子、中子和电子的数量计算？

📚 How Many Protons, Neutrons and Electrons? | 质子、中子和电子的数量计算？

For A‑Level Chemistry, one of the most fundamental skills is determining the number of protons, neutrons and electrons in an atom or ion. Whether you are looking at a neutral atom, a charged ion or an isotope, the rules are simple once you understand atomic number and mass number. This article walks you through every concept, from the definition of subatomic particles to worked examples using nuclide notation, helping you avoid common mistakes and build confidence for your Cambridge exams.

在A-level化学中，最基本的一项技能就是确定一个原子或离子中的质子、中子和电子数目。无论是中性原子、带电荷的离子还是同位素，只要理解了原子序数和质量数，规则其实非常简单。本文将带你逐一梳理每个概念，从亚原子粒子的定义，到使用核素符号进行实际演练，帮助你避开常见错误，为剑桥考试建立十足的信心。

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1. Atomic Number – The Identity of an Element | 原子序数——元素的身份

The atomic number, symbol Z, is the number of protons in the nucleus of an atom. It is unique for every element and determines the element’s position in the Periodic Table. For example, carbon always has Z = 6, oxygen has Z = 8, and sodium has Z = 11. You cannot change the atomic number without changing the element itself.

原子序数，符号 Z，是原子核中质子的数量。每一种元素都有独一无二的原子序数，并决定了它在周期表中的位置。例如，碳的 Z 恒为 6，氧为 8，钠为 11。如果改变了原子序数，元素本身也就改变了。

In a neutral atom, the atomic number also tells you how many electrons surround the nucleus. This is because the positive charge of the protons must be balanced by an equal number of negatively charged electrons. Therefore, knowing Z immediately gives you both the proton count and, in a neutral atom, the electron count.

在中性原子中，原子序数也告诉你核外有多少个电子。这是因为质子的正电荷必须被等量的带负电的电子所平衡。因此，知道了 Z，就立刻知道了质子的数量，并且在中性原子中也知道了电子的数量。

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2. Protons – Always Equal to the Atomic Number | 质子——总是等于原子序数

The number of protons is the defining feature of an element and never varies in a chemical reaction. Protons carry a single positive charge and reside in the nucleus. Their number is always exactly equal to the atomic number, Z. For any atom or ion of a given element, the proton count remains unchanged. For example, every atom of chlorine contains exactly 17 protons, whether it is a neutral atom or a chloride ion, Cl⁻.

质子的数量是元素的决定性特征，在化学反应中永不改变。质子带一个正电荷，位于原子核内。其数量始终精确等于原子序数 Z。对于某种元素的任何原子或离子，质子数目保持不变。例如，每一个氯原子都恰好含有 17 个质子，无论它是中性原子还是氯离子 Cl⁻。

This constancy makes protons an excellent starting point for all subatomic particle calculations. Once you know the element, you know the number of protons. No further calculation is needed, and you can immediately write down this value from a Periodic Table.

这种恒定性使质子成为所有亚原子粒子计算的绝佳起点。一旦知道了元素，就知道了质子数。无需进一步计算，你就可以立刻从周期表中查出并写下这个数值。

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3. Mass Number – The Sum of Protons and Neutrons | 质量数——质子与中子之和

The mass number, symbol A, is the total number of protons and neutrons in the nucleus. It is always a whole number and is often given together with the element symbol. For instance, ¹²C has a mass number of 12, meaning the nucleus contains 12 protons + neutrons. Because protons and neutrons each have a relative mass of approximately 1, the mass number approximates the atom’s relative atomic mass in atomic mass units.

质量数，符号 A，是原子核中质子与中子的总数。它总是一个整数，通常和元素符号一起给出。例如，¹²C 的质量数为 12，意味着原子核含有 12 个质子与中子之和。因为质子和中子的相对质量都约为 1，质量数近似等于以原子质量单位表示的原子相对质量。

It is crucial to distinguish mass number from atomic number. While Z tells you the number of protons and fixes the element’s identity, A tells you about the particular isotope. The relationship A = protons + neutrons is key to finding neutron numbers.

区分质量数与原子序数至关重要。Z 告诉你质子数并确定元素身份，而 A 则告诉你具体是哪种同位素。关系式 A = 质子数 + 中子数 是求解中子数的关键。

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4. Neutrons – Calculating from Mass Number and Atomic Number | 中子——由质量数和原子序数计算

The number of neutrons in an atom is not fixed for an element, because isotopes exist. Neutrons carry no charge and are located in the nucleus. To find the number of neutrons, you simply subtract the atomic number from the mass number: number of neutrons = A − Z.

原子中的中子数对一种元素来说并不是固定的，因为有同位素的存在。中子不带电，位于原子核内。求中子数时，只需用质量数减去原子序数：中子数 = A − Z。

For example, in an atom of ²³₁₁Na (sodium-23), A = 23, Z = 11, so neutrons = 23 − 11 = 12. For ³⁷₁₇Cl (chlorine-37), Z = 17, A = 37, giving 20 neutrons. This simple subtraction works for any atom, regardless of whether it is neutral or charged, because ions are formed by the loss or gain of electrons, not neutrons.

例如，在一个 ²³₁₁Na（钠-23）原子中，A=23，Z=11，因此中子数 = 23 − 11 = 12。对于 ³⁷₁₇Cl（氯-37），Z=17，A=37，得到 20 个中子。这个简单的减法对任何原子都适用，不管它是中性还是带电的，因为离子是通过失去或得到电子而形成的，而不是中子。

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5. Electrons in Neutral Atoms | 中性原子中的电子

In an electrically neutral atom, the number of electrons is exactly equal to the number of protons. Electrons are negatively charged and arranged in shells around the nucleus. For every element, a neutral atom contains Z electrons. So for magnesium (Z=12), a neutral Mg atom has 12 electrons; for bromine (Z=35), a neutral Br atom has 35 electrons.

在电中性的原子中，电子数精确等于质子数。电子带负电，排列在原子核外的电子层中。对于每种元素，中性原子含有 Z 个电子。因此，镁（Z=12）的中性原子有 12 个电子；溴（Z=35）的中性原子有 35 个电子。

This equality arises because the total positive charge from Z protons is balanced by Z electrons. As soon as you see the word ‘atom’ without any charge indicated, assume the number of electrons equals the atomic number.

这种相等关系是因为 Z 个质子带来的总正电荷恰好被 Z 个电子所平衡。每当你看到没有标注任何电荷的“原子”时，就可以认定电子数等于原子序数。

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6. Ions – Adjusting Electrons for Charge | 离子——根据电荷调整电子数

When an atom gains or loses electrons, it becomes an ion. A positive ion (cation) has fewer electrons than protons, while a negative ion (anion) has more electrons than protons. The charge on an ion is shown as a superscript, such as Na⁺, Mg²⁺, O²⁻, or Cl⁻. The magnitude of the charge tells you how many electrons have been lost (positive charge) or gained (negative charge).

当原子得到或失去电子时，它就变成了离子。阳离子（cation）的电子数少于质子数，而阴离子（anion）的电子数多于质子数。离子的电荷以上标形式表示，如 Na⁺、Mg²⁺、O²⁻ 或 Cl⁻。电荷的数值告诉你失去了多少个电子（正电荷）或得到了多少个电子（负电荷）。

To calculate the number of electrons in an ion, start with the atomic number Z, then subtract the charge (algebraically). For a cation with charge +n, electrons = Z − n. For an anion with charge −n, electrons = Z + n. For example, Al³⁺: Z = 13, electrons = 13 − 3 = 10. For S²⁻: Z = 16, electrons = 16 + 2 = 18. Note that the number of protons and neutrons remains unchanged from the neutral atom.

计算离子中的电子数时，从原子序数 Z 开始，然后减去电荷（代数值）。对于电荷为 +n 的阳离子，电子数 = Z − n；对于电荷为 −n 的阴离子，电子数 = Z + n。例如，Al³⁺：Z=13，电子数 = 13−3=10。S²⁻：Z=16，电子数 = 16+2=18。注意，质子数和中子数与中性原子相比保持不变。

This adjustment is essential for writing electron configurations of ions and for explaining periodic trends such as ionic radii. Always remember: ion charge only affects the electron count, never the proton or neutron count.

这个调整对于书写离子的电子排布以及解释离子半径等周期趋势至关重要。务必牢记：离子电荷只影响电子数目，绝不会影响质子数或中子数。

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7. Nuclide Notation – How to Represent an Atom or Ion | 核素符号——如何表示原子或离子

Nuclide notation is a compact way to display an atom or ion’s mass number, atomic number, and charge. The mass number A is written as a left superscript, the atomic number Z as a left subscript, and the charge as a right superscript. For example, the sodium cation is written as ²³₁₁Na⁺, and the oxide ion as ¹⁶₈O²⁻.

核素符号是一种紧凑的方式，用来显示原子或离子的质量数、原子序数和电荷。质量数 A 写在左上标位置，原子序数 Z 写在左下标位置，电荷写在右上标位置。例如，钠离子写成 ²³₁₁Na⁺，氧离子写成 ¹⁶₈O²⁻。

Sometimes Z is omitted because the element symbol already implies the atomic number. However, for exam purposes, you should be able to read and construct nuclide notation with all three pieces of information. When the charge is not shown, the species is a neutral atom.

有时 Z 会被省略，因为元素符号本身已经隐含了原子序数。但为了考试，你应该能够读取并构造包含全部三项信息的核素符号。当电荷没有显示时，该物种就是一个中性原子。

Using nuclide notation, you can instantly determine:

• Protons = Z
• Neutrons = A − Z
• Electrons = Z − (charge shown)

This systematic approach reduces errors in multi‑step problems.

利用核素符号，你可以立刻确定：

• 质子数 = Z
• 中子数 = A − Z
• 电子数 = Z −（所标示的电荷）

这种系统的方法可以减少多步题目中的错误。

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8. Isotopes – Same Proton Number, Different Neutron Number | 同位素——相同质子数，不同中子数

Isotopes are atoms of the same element (same Z) that have different mass numbers due to different numbers of neutrons. For example, carbon has three naturally occurring isotopes: ¹²₆C (6 neutrons), ¹³₆C (7 neutrons), and ¹⁴₆C (8 neutrons). All of them contain 6 protons, so they belong to carbon. The chemical properties of isotopes are nearly identical because they have the same electron configuration, but their physical masses and nuclear stabilities differ.

同位素是同一种元素（Z 相同）的原子，由于中子数不同而具有不同的质量数。例如，碳有三种天然同位素：¹²₆C（6 个中子）、¹³₆C（7 个中子）和 ¹⁴₆C（8 个中子）。它们都含有 6 个质子，因此都属于碳元素。同位素的化学性质几乎完全相同，因为它们具有相同的电子排布，但物理质量和核稳定性有所不同。

When you are asked to work out numbers of subatomic particles for a specific isotope, you must use the given mass number A. Never use the average relative atomic mass from the Periodic Table for these calculations, as that value is a weighted average that includes all isotopes.

当你被要求计算某个特定同位素的亚原子粒子数目时，必须使用题目给出的质量数 A。千万不要把周期表上的平均相对原子质量用于这些计算，因为那个数值是包含所有同位素的加权平均值。

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9. Worked Examples – From Nuclide Notation to Subatomic Counts | 实例演练——从核素符号到亚原子粒子计数

Let’s work through several examples using nuclide notation to find protons, neutrons, and electrons. This step‑by‑step method will prepare you for typical examination questions.

让我们通过几个使用核素符号的实例来找出质子、中子和电子数目。这种分步方法将帮助你为典型的考试题目做好准备。

Example 1: ³²₁₆S²⁻
Protons = Z = 16.
Neutrons = A − Z = 32 − 16 = 16.
Electrons: neutral S would have 16 electrons; the 2− charge means two extra electrons, so electrons = 16 + 2 = 18.

例1：³²₁₆S²⁻
质子数 = Z = 16。
中子数 = A − Z = 32 − 16 = 16。
电子数：中性 S 有 16 个电子；2− 电荷表示多了两个电子，所以电子数 = 16 + 2 = 18。

Example 2: ⁴⁰₂₀Ca²⁺
Protons = 20.
Neutrons = 40 − 20 = 20.
Electrons = 20 − 2 = 18.

例2：⁴⁰₂₀Ca²⁺
质子数 = 20。
中子数 = 40 − 20 = 20。
电子数 = 20 − 2 = 18。

Example 3: ¹⁹₉F⁻
Protons = 9.
Neutrons = 19 − 9 = 10.
Electrons = 9 + 1 = 10.

例3：¹⁹₉F⁻
质子数 = 9。
中子数 = 19 − 9 = 10。
电子数 = 9 + 1 = 10。

As these examples show, a logical order – first protons, then neutrons, then electrons – makes even complicated‑looking symbols easy to decode.

正如这些例子所示，按照逻辑顺序——先质子，再中子，后电子——即使看起来复杂的符号也很容易解读。

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10. Common Pitfalls and Misconceptions | 常见陷阱与误解

Students often confuse mass number (A) with atomic number (Z), leading to incorrect neutron calculations. Remember: A is always the larger number given on the top left, and Z is the smaller bottom left number. When only one number is shown, such as in “chlorine-35,” that number is the mass number, not the atomic number.

学生经常将质量数（A）和原子序数（Z）搞混，从而导致中子计算错误。请记住：A 总是左上方的较大数字，Z 是左下方较小的数字。当只显示一个数字时，例如“氯-35”，那个数字是质量数，而不是原子序数。

Another frequent error is forgetting to adjust the electron count for ions. Writing the electron configuration of Na⁺ as 2,8,1 (11 electrons) instead of 2,8 (10 electrons) is a classic mistake. Always ask yourself: has the species lost or gained electrons? Use the charge to decide.

另一个常见错误是忘记为离子调整电子数目。将 Na⁺ 的电子排布写成 2,8,1（11 个电子）而不是 2,8（10 个电子）是典型的错误。一定要问自己：这个物种是失去了还是得到了电子？用电荷来判断。

Finally, do not use the relative atomic mass from the Periodic Table to calculate neutrons unless the question specifically asks for the number of neutrons in a specific isotope. The relative atomic mass is a weighted average and is often not a whole number, so it will give you a misleading result.

最后，不要用周期表上的相对原子质量来计算中子数，除非题目明确要求计算某一特定同位素的中子数。相对原子质量是加权平均值，通常不是整数，因此会得出错误的结果。

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11. Practice Questions and Step-by-Step Approach | 练习题与分步方法

Below is a table of species for you to practise. Use a step‑by‑step strategy: first identify Z from the symbol or element name, then A, then the charge, and finally compute the three counts.

下面是一个练习物种的表格，供你练习。采用分步策略：先从符号或元素名称中确定 Z，然后确定 A，再确定电荷，最后计算三个粒子数目。

Species	Protons	Neutrons	Electrons
²⁴₁₂Mg	12	12	12
⁵⁶₂₆Fe³⁺	26	30	23
⁷⁹₃₅Br⁻	35	44	36
³¹₁₅P³⁻	15	16	18

Your internal checklist should be: what is the element? (gives Z) → what isotope? (gives A) → is there a charge? (adjust electrons). With practice, this becomes automatic and fast, saving valuable time in the exam.

你的内部检查表应该是：这是什么元素？（给出 Z）→ 哪种同位素？（给出 A）→ 有没有电荷？（调整电子数）。通过练习，这会变得自动而快速，为考试节省宝贵的时间。

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12. Summary and Key Points | 总结与要点

Understanding protons, neutrons and electrons is foundational for mastering atomic structure, bonding, periodicity and ionisation energies. The key points to remember are:

• Protons = atomic number Z (always constant for an element).
• Neutrons = mass number A − Z.
• Electrons = Z (neutral atom) or Z − charge (for ions).
• Isotopes differ only in neutron number; their proton and electron numbers are identical.
• Always use the specific mass number given, never the relative atomic mass from the Periodic Table.

If you follow a consistent, step‑by‑step method and keep these points in mind, you will be able to answer any related question quickly and accurately.

理解质子、中子和电子是掌握原子结构、化学键、周期性和电离能的基础。需要记住的关键点是：

• 质子数 = 原子序数 Z（对一种元素始终恒定）。
• 中子数 = 质量数 A − Z。
• 电子数 = Z（中性原子）或 Z − 电荷（离子）。
• 同位素只在中子数上有差异，它们的质子数和电子数完全相同。
• 始终使用题目所给的特定质量数，切勿使用周期表上的相对原子质量。

如果你遵循一致的、分步的方法并牢记这些要点，就能快速而准确地回答任何相关题目。

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Published by TutorHao | Chemistry Revision Series | aleveler.com

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