How to Determine Protons, Neutrons, and Electrons | A-Level化学：如何确定质子、中子与电子数

📚 How to Determine Protons, Neutrons, and Electrons | A-Level化学：如何确定质子、中子与电子数

Understanding how to determine the number of protons, neutrons, and electrons in an atom or ion is a fundamental skill in A-Level Chemistry. This knowledge forms the basis for interpreting the periodic table, predicting chemical behaviour, and understanding isotopic notation.

在A-Level化学中，确定原子或离子中质子、中子与电子的数目是一项基础核心技能。掌握这一知识是解读元素周期表、预测化学性质以及理解同位素表示法的前提。

1. The Nuclear Symbol Notation | 核素符号表示法

The nuclear symbol (also called nuclide notation) is a shorthand way of writing the composition of an atom. It uses the mass number and atomic number placed around the element’s symbol. The mass number (A) is written as a superscript to the upper left, while the atomic number (Z) is written as a subscript to the lower left.

核素符号（又称核素表示法）是书写原子组成的简写方式。它在元素符号的左上角标注质量数（A），在左下角标注原子序数（Z）。

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X = chemical symbol of the element | X = 元素的化学符号
A = mass number (protons + neutrons) | A = 质量数（质子数 + 中子数）
Z = atomic number (number of protons) | Z = 原子序数（质子数）

For example, the notation ²³₁₁Na tells us the atom has 11 protons and a mass number of 23. The number of neutron is then calculated as A − Z = 23 − 11 = 12.

例如，符号 ²³₁₁Na 表示该原子含有11个质子和质量数23。中子数可通过 A − Z = 23 − 11 = 12 计算得出。

2. The Atomic Number (Z) and Proton Number | 原子序数（Z）与质子数

The atomic number, represented by Z, is defined as the number of protons in the nucleus of an atom. In a neutral atom, the number of protons equals the number of electrons, making the atom electrically neutral.

原子序数（符号Z）定义为原子核中的质子数目。在电中性原子中，质子数等于电子数，因此原子整体不带电。

Key points to remember:

Z is unique for each element: all carbon atoms have Z = 6, meaning 6 protons. | Z 对每种元素是唯一的：所有碳原子的 Z = 6，即有6个质子。
Protons define the element’s identity. | 质子数目决定元素的种类。
In the periodic table, elements are arranged in order of increasing atomic number. | 周期表中元素按原子序数递增排列。

3. The Mass Number (A) and Neutron Number | 质量数（A）与中子数

The mass number (A) is the total number of protons and neutrons in the nucleus. Since each proton and neutron has a mass of approximately 1 atomic mass unit (amu), the mass number is always a whole number.

质量数（A）是原子核中质子与中子的总数。由于每个质子和中子的质量约为1个原子质量单位（amu），质量数始终为整数。

Number of neutrons = A − Z

中子数 = A − Z

For example, for ⁴⁰₁₈Ar: neutrons = 40 − 18 = 22. For ³⁵₁₇Cl: neutrons = 35 − 17 = 18.

例如，⁴⁰₁₈Ar 的中子数 = 40 − 18 = 22；³⁵₁₇Cl 的中子数 = 35 − 17 = 18。

4. Isotopes and Their Notation | 同位素与其表示法

Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons. Since they have the same number of protons, they share the same atomic number and chemical properties, but differ in mass number.

同位素是指同一种元素中具有相同质子数但不同中子数的原子。由于质子数相同，它们具有相同的原子序数和化学性质，但质量数不同。

Isotope 同位素	Protons 质子数	Neutrons 中子数	Mass Number 质量数
¹²₆C	6	6	12
¹⁴₆C	6	8	14
³⁵₁₇Cl	17	18	35
³⁷₁₇Cl	17	20	37

5. Determining Electrons in Neutral Atoms | 中性原子中的电子数确定

In a neutral atom, the number of electrons is exactly equal to the number of protons. This is because the positive charges of the protons must balance the negative charges of the electrons.

在中性原子中，电子数恰好等于质子数。这是因为质子的正电荷必须与电子的负电荷相平衡。

Number of electrons = number of protons = Z (for neutral atoms)

电子数 = 质子数 = Z（对于中性原子）

For example, a neutral calcium atom (Z = 20) has 20 protons, 20 electrons, and if its mass number is 40, it has 20 neutrons.

例如，一个中性钙原子（Z = 20）有20个质子、20个电子；若其质量数为40，则有20个中子。

6. Determining Electrons in Ions | 离子中电子数的确定

Ions are atoms or groups of atoms that carry a positive or negative charge due to a loss or gain of electrons. The charge on an ion provides crucial information for determining the number of electrons.

离子是原子或原子团因失去或获得电子而带有正电荷或负电荷。离子所带的电荷数提供了确定电子数的关键信息。

For a cation (positively charged ion):

Electrons = protons − charge magnitude | 电子数 = 质子数 − 电荷数值
Example: Na⁺ has 11 protons and a 1+ charge, so electrons = 11 − 1 = 10. | 例：Na⁺ 有11个质子和1个正电荷，因此电子数 = 11 − 1 = 10。

For an anion (negatively charged ion):

Electrons = protons + charge magnitude | 电子数 = 质子数 + 电荷数值
Example: Cl⁻ has 17 protons and a 1− charge, so electrons = 17 + 1 = 18. | 例：Cl⁻ 有17个质子和1个负电荷，因此电子数 = 17 + 1 = 18。

Electrons = Z − (charge value), where charge is + for cations and − for anions

电子数 = Z −（电荷数值），阳离子取正，阴离子取负

7. Worked Examples: Full Calculations | 完整计算示例

Let us apply these rules to complete problems involving different species.

下面我们通过完整例题来应用以上规则。

Example 1: ⁵⁶₂₆Fe²⁺

Protons = Z = 26 | 质子数 = Z = 26
Neutrons = A − Z = 56 − 26 = 30 | 中子数 = A − Z = 56 − 26 = 30
Electrons = 26 − 2 = 24 (because it is a 2+ ion) | 电子数 = 26 − 2 = 24（因为带2+电荷）

Example 2: ³²₁₆S²⁻

Protons = 16 | 质子数 = 16
Neutrons = 32 − 16 = 16 | 中子数 = 32 − 16 = 16
Electrons = 16 + 2 = 18 (because it is a 2− ion) | 电子数 = 16 + 2 = 18（因为带2−电荷）

8. Common Mistakes to Avoid | 常见错误防范

Students often make errors when determining subatomic particles. Being aware of these common pitfalls will help you perform confidently in exams.

学生在确定亚原子粒子时经常出错。了解这些常见误区有助于你在考试中自信作答。

Confusing mass number with atomic number. Remember: A = p + n, not A = p only. | 混淆质量数与原子序数。切记：A = p + n，而非 A = p。
Forgetting to adjust electron count for ions. Neutral-atom electron counts do not apply to ions. | 忘记对离子进行电子数调整。中性原子的电子规则不适用于离子。
Adding the charge to the wrong side. For a positive ion, subtract; for a negative ion, add. | 电荷加减方向搞反。正离子要减，负离子要加。
Misreading the superscript/subscript in nuclear notation. | 误读核素符号中的上标和下标。

9. The Relationship Between Particles and Periodic Table Position | 粒子数与周期表位置的关系

The number of protons (atomic number) directly determines the element’s position in the periodic table. The period number corresponds to the number of electron shells, while the group number is related to the number of electrons in the outer shell (for main-group elements).

质子数（原子序数）直接决定元素在周期表中的位置。周期数对应电子壳层数，而族号与最外层电子数相关（对于主族元素）。

For example, magnesium (Z = 12) is in Period 3 and Group 2. Its electron arrangement is 2, 8, 2, meaning it has 2 electrons in its outer shell. This electron configuration is derived directly from its proton number.

例如，镁（Z = 12）位于第三周期、第二族。其电子排布为2, 8, 2，意味着最外层有2个电子。这一电子构型直接由其质子数推导得出。

10. Electron Configurations from Particle Counts | 从粒子数推导电子构型

Once you know the number of electrons in an atom or ion, you can write its electronic configuration. The order of shell filling follows the Aufbau principle: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

一旦你知道了原子或离子中的电子数目，就可以写出其电子构型。电子填充顺序遵循构造原理：1s、2s、2p、3s、3p、4s、3d、4p 等。

For ions, remember that electrons are lost from the outer shell first (the highest principal quantum number n). For example, Fe²⁺ has 24 electrons, giving the configuration [Ar] 3d⁶. The two 4s electrons are removed before the 3d electrons.

对于离子，记住电子最先从最外层（主量子数n最大）失去。例如，Fe²⁺ 有24个电子，其构型为 [Ar] 3d⁶。两个4s电子比3d电子更先失去。

11. Exam-Style Practice Questions | 考试风格练习题

Test your understanding with these typical A-Level questions.

用这些典型A-Level题目测试你的理解。

Question 1: Determine the number of protons, neutrons, and electrons in ⁷⁵₃₃As³⁻.

问题1：确定 ⁷⁵₃₃As³⁻ 中的质子数、中子数和电子数。

Answer: Protons = 33, neutrons = 75 − 33 = 42, electrons = 33 + 3 = 36.

答案：质子数 = 33，中子数 = 75 − 33 = 42，电子数 = 33 + 3 = 36。

Question 2: An ion has 20 protons, 22 neutrons, and 18 electrons. Write its nuclear symbol and identify the charge.

问题2：某离子含20个质子、22个中子和18个电子。写出其核素符号并判断其所带电荷。

Answer: Protons = 20 means Ca. Electrons = 18, which is 2 fewer than the protons, so the charge is 2+. Mass number = 20 + 22 = 42. Therefore the symbol is ⁴²₂₀Ca²⁺.

答案：质子数为20意味着该元素是Ca。电子数 = 18，比质子数少2个，因此带2+电荷。质量数 = 20 + 22 = 42。因此核素符号为 ⁴²₂₀Ca²⁺。

12. Summary of Rules | 规则总结

Here is a quick reference table summarising how to determine the number of each subatomic particle.

以下是快速参考表，总结如何确定每种亚原子粒子的数目。

Quantity 项目	Formula 公式	Key Note 要点
Protons 质子数	= Z	Always equals atomic number 始终等于原子序数
Neutrons 中子数	= A − Z	Can vary within isotopes 在同位素中可以不同
Electrons (atom) 电子数（原子）	= Z	Neutral species only 仅适用于中性物种
Electrons (cation) 电子数（阳离子）	= Z − charge	Subtract the positive charge 减去正电荷量
Electrons (anion) 电子数（阴离子）	= Z + charge	Add the negative charge 加上负电荷量

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How to Determine Protons, Neutrons, and Electrons | A-Level化学：如何确定质子、中子与电子数