📚 Finding Electrons: How to Locate and Count Electrons in Atoms | 寻找电子：如何定位和计算原子中的电子
Electrons are the tiny, negatively charged particles that surround the nucleus of an atom. Finding electrons is not about searching with a microscope; it is about using the periodic table, the atomic number, and simple shell rules to work out how many electrons an atom has and where those electrons are arranged.
电子是围绕原子核的微小带负电粒子。寻找电子并不是用显微镜去搜索，而是利用元素周期表、原子序数和简单的壳层规则来算出原子有多少个电子，以及这些电子如何排布。
1. What are electrons? | 电子是什么？
Electrons are subatomic particles with a negative charge. They are much lighter than protons and neutrons, and they move rapidly in the space around the nucleus. The symbol for an electron is e⁻.
电子是带负电的亚原子粒子。它们比质子和中子轻得多，并且在原子核周围的空间中快速运动。电子的符号是 e⁻。
Because electrons have a charge of -1, they are attracted to the positively charged protons in the nucleus. This attraction is one of the key forces that holds an atom together.
由于电子带有 -1 的电荷，它们会被原子核中带正电的质子吸引。这种吸引力是将原子维系在一起的关键作用力之一。
2. Where do electrons live in an atom? | 电子在原子中居住在哪里？
Electrons do not move randomly inside an atom. They are arranged in energy levels, also called shells, around the nucleus. The nucleus contains protons and neutrons, while the electrons occupy the space outside the nucleus.
电子并不是在原子内部随机运动。它们排列在围绕原子核的能级中，这些能级也称为电子壳层。原子核包含质子和中子，而电子则占据原子核外的空间。
The shells are like layers of an onion. The shell closest to the nucleus has the lowest energy, and shells further away have higher energy. Electrons fill the lowest energy shells first before moving to higher ones.
电子壳层就像洋葱的层。最靠近原子核的壳层能量最低，离得越远的壳层能量越高。电子会先填满能量最低的壳层，然后再进入能量更高的壳层。
3. The nuclear model and electron shells | 核式模型与电子壳层
The modern nuclear model of the atom shows a tiny, dense nucleus surrounded by electrons in shells. Most of the atom is empty space, which is why electrons can move around so freely.
现代原子的核式模型显示，一个微小而致密的原子核被壳层中的电子包围。原子的大部分是空的空间，这就是电子能够如此自由运动的原因。
In KS3 chemistry, we usually draw electron shells as circles around the nucleus. The nucleus is labelled with protons and neutrons, and the shells are filled with small dots or crosses to represent electrons.
在 KS3 化学中，我们通常把电子壳层画成围绕原子核的圆圈。原子核标注质子和中子，壳层中用小点或叉号来表示电子。
4. Reading the periodic table for electron clues | 从元素周期表读取电子线索
The periodic table gives you the fastest way to find the number of electrons in a neutral atom. Each element has an atomic number, which is usually written above the element symbol.
元素周期表为你提供了找到中性原子中电子数的最快方法。每种元素都有一个原子序数，通常写在元素符号的上方。
For example, carbon has an atomic number of 6. This means a neutral carbon atom has 6 protons in its nucleus and 6 electrons around the nucleus.
例如，碳的原子序数是 6。这意味着一个中性碳原子的原子核中有 6 个质子，核外有 6 个电子。
5. Atomic number = proton number = electron number | 原子序数 = 质子数 = 电子数
For a neutral atom, the number of electrons is exactly equal to the number of protons. This is because the positive charge of each proton must be balanced by the negative charge of each electron.
对于一个中性原子，电子数正好等于质子数。这是因为每个质子的正电荷必须被每个电子的负电荷所平衡。
Number of electrons in a neutral atom = number of protons = atomic number (Z)
So if you know the atomic number of an element, you immediately know how many electrons a neutral atom of that element has. For instance, oxygen has an atomic number of 8, so it has 8 electrons.
因此，如果你知道一种元素的原子序数，你就能立刻知道该元素中性原子的电子数。例如，氧的原子序数是 8，所以它有 8 个电子。
6. Electron shell filling rules: 2, 8, 8 | 电子壳层填充规则：2、8、8
Electrons fill shells in a specific order. At KS3 level, the first shell can hold up to 2 electrons, the second shell can hold up to 8 electrons, and the third shell can hold up to 8 electrons.
电子按照特定顺序填充壳层。在 KS3 阶段，第一壳层最多容纳 2 个电子，第二壳层最多容纳 8 个电子，第三壳层最多容纳 8 个电子。
- First shell: up to 2 electrons | 第一壳层：最多 2 个电子
- Second shell: up to 8 electrons | 第二壳层：最多 8 个电子
- Third shell: up to 8 electrons at KS3 | 第三壳层：KS3 阶段最多 8 个电子
This is often called the 2,8,8 rule. It works well for the first 20 elements, which are the main focus at KS3.
这通常被称为 2、8、8 规则。它适用于前 20 号元素，这也是 KS3 阶段的主要学习内容。
7. Worked example: sodium | 实例：钠
Sodium has an atomic number of 11, so a neutral sodium atom has 11 electrons. These electrons are arranged by filling the shells in order: 2 in the first shell, 8 in the second shell, and the remaining 1 in the third shell.
钠的原子序数是 11，因此一个中性钠原子有 11 个电子。这些电子按顺序填充壳层：第一壳层 2 个，第二壳层 8 个，剩下的 1 个在第三壳层。
Na: 2,8,1
The single electron in the outer shell makes sodium very reactive. Metals like sodium often have only 1, 2 or 3 electrons in their outer shell.
最外层唯一的电子使钠非常活泼。像钠这样的金属通常在最外层只有 1、2 或 3 个电子。
8. Worked example: chlorine | 实例：氯
Chlorine has an atomic number of 17, so a neutral chlorine atom has 17 electrons. Using the 2,8,8 rule, the electrons fill the shells as 2 in the first shell, 8 in the second shell, and 7 in the third shell.
氯的原子序数是 17，因此一个中性氯原子有 17 个电子。根据 2、8、8 规则，电子填充壳层为：第一壳层 2 个，第二壳层 8 个，第三壳层 7 个。
Cl: 2,8,7
Chlorine needs one more electron to fill its outer shell. This explains why chlorine is a very reactive non-metal.
氯还需要一个电子才能填满最外层。这就解释了为什么氯是一种非常活泼的非金属。
9. Ions: when atoms gain or lose electrons | 离子：原子得到或失去电子时
An ion is an atom or group of atoms that has gained or lost electrons. When an atom loses electrons, it becomes a positive ion. When an atom gains electrons, it becomes a negative ion.
离子是得到或失去电子的原子或原子团。当原子失去电子时，它变成正离子。当原子得到电子时，它变成负离子。
For example, a sodium atom loses 1 electron to form a sodium ion. A sodium ion has 11 protons but only 10 electrons, so its charge is 1+.
例如，钠原子失去 1 个电子形成钠离子。钠离子有 11 个质子，但只有 10 个电子，因此它的电荷为 1+。
Na⁺: 2,8 | Cl⁻: 2,8,8
A chlorine atom gains 1 electron to form a chloride ion. The chloride ion has 17 protons and 18 electrons, so its charge is 1-.
氯原子得到 1 个电子形成氯离子。氯离子有 17 个质子和 18 个电子，因此它的电荷为 1-。
10. Isotopes and electron count | 同位素与电子数
Isotopes are atoms of the same element with different numbers of neutrons. However, they always have the same number of protons, so they also have the same number of electrons in a neutral atom.
同位素是同一种元素中中子数不同的原子。但是，它们总是具有相同的质子数，因此中性原子中的电子数也相同。
For example, carbon-12 and carbon-14 are both isotopes of carbon. Both have 6 protons and therefore 6 electrons, even though carbon-14 has two extra neutrons.
例如，碳-12 和碳-14 都是碳的同位素。两者都有 6 个质子，因此都有 6 个电子，尽管碳-14 多了两个中子。
This shows that finding electrons depends on the atomic number, not the mass number. Neutrons do not affect the electron count.
这表明寻找电子取决于原子序数，而不是质量数。中子不影响电子数。
11. Electron diagrams (dot-and-cross) | 电子图（点叉图）
Electron diagrams use dots and crosses to show how electrons are arranged in shells. You draw the nucleus in the centre, then add electron shells around it and place the correct number of dots or crosses in each shell.
电子图使用点和叉来显示电子在壳层中的排列方式。你在中心画出原子核，然后在周围添加电子壳层，并在每个壳层中放置正确数量的点或叉。
For nitrogen, which has an atomic number of 7, you draw 2 electrons in the first shell and 5 electrons in the second shell. The electron arrangement is 2,5.
对于原子序数为 7 的氮，你在第一壳层画 2 个电子，在第二壳层画 5 个电子。电子排布为 2,5。
N: 2,5
Dot-and-cross diagrams are useful for showing the outer shell electrons, which are the electrons involved in chemical bonding.
点叉图对于展示最外层电子非常有用，这些电子正是参与化学键合的电子。
12. Common misconceptions and quick checks | 常见误区与快速自测
One common misconception is that electrons are always equal to protons. This is only true for neutral atoms. In ions, the electron number changes because electrons are gained or lost.
一个常见的误区是电子数总是等于质子数。这仅对中性原子成立。在离子中，由于电子被得到或失去，电子数会发生变化。
Another misconception is that electrons sit still in shells. In reality, electrons move constantly, but shell diagrams show the most likely energy levels where they can be found.
另一个误区是电子静止地待在壳层中。实际上，电子不断运动，但壳层图展示的是它们最可能所处的能级。
Quick check: a neutral magnesium atom has an atomic number of 12. How many electrons does it have, and what is its electron arrangement?
快速自测：一个中性镁原子的原子序数是 12。它有多少个电子？它的电子排布是什么？
Mg: 2,8,2
Magnesium has 12 electrons, arranged as 2 in the first shell, 8 in the second shell, and 2 in the third shell. This follows the 2,8,8 rule.
镁有 12 个电子，排布为第一壳层 2 个，第二壳层 8 个，第三壳层 2 个。这符合 2、8、8 规则。
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