📚 Atomic Structure | 原子结构
The atom is the smallest unit of an element that retains the chemical properties of that element. A thorough understanding of atomic structure is a cornerstone of IGCSE Science and unlocks the behaviour of materials, chemical reactions and the periodic table.
原子是保持元素化学性质的最小单位。透彻理解原子结构是IGCSE科学的基础，也是理解物质行为、化学反应和元素周期表的关键。
1. Atoms and Elements | 原子与元素
All matter is composed of atoms. An element is a pure substance made of only one type of atom. Each element is represented by a unique chemical symbol, such as C for carbon, O for oxygen and Na for sodium.
所有物质都由原子组成。元素是由一种原子组成的纯净物质。每种元素都有唯一的化学符号，例如碳用C表示，氧用O表示，钠用Na表示。
Atoms are extremely small; a single drop of water contains over a billion billion atoms. The concept of atoms was proposed by John Dalton in the early 19th century, but it was not until the 20th century that experiments revealed their detailed internal structure.
原子极小；一滴水中含有超过十亿亿个原子。原子概念由约翰·道尔顿在19世纪初提出，但直到20世纪，实验才揭示了其详细的内部结构。
2. Subatomic Particles | 亚原子粒子
Atoms consist of three main subatomic particles: protons, neutrons and electrons. Protons and neutrons are found in the nucleus, while electrons move around the nucleus in regions called electron shells.
原子由三种主要亚原子粒子组成：质子、中子和电子。质子和中子位于原子核内，而电子在称为电子壳层的区域中围绕核运动。
The properties of these particles are summarised in the table below:
这些粒子的性质如下表所示：
| Particle 粒子 | Relative mass 相对质量 | Relative charge 相对电荷 | Location 位置 |
| Proton 质子 | 1 | +1 | Nucleus 原子核 |
| Neutron 中子 | 1 | 0 | Nucleus 原子核 |
| Electron 电子 | 1/1836 (negligible) | −1 | Electron shells 电子壳层 |
Protons are positively charged, electrons are negatively charged, and neutrons have no charge. Since atoms are neutral, the number of protons equals the number of electrons.
质子带正电，电子带负电，中子不带电。由于原子是电中性的，质子数等于电子数。
3. Atomic Number and Mass Number | 原子序数与质量数
Every element has a unique atomic number (Z), which is the number of protons in its nucleus. The mass number (A) is the total number of protons and neutrons.
每种元素都有唯一的原子序数（Z），即其原子核中的质子数。质量数（A）是质子和中子的总数。
This relationship can be expressed as:
这种关系可以表示为：
A = Z + N
where N is the number of neutrons. The number of neutrons can therefore be found using N = A − Z.
其中N是中子数。因此，中子数可以通过 N = A − Z 求得。
In standard notation, the mass number is written as a superscript before the symbol and the atomic number as a subscript, for example: 12₆C.
在标准表示法中，质量数写在元素符号的左上方，原子序数写在左下方，例如：12₆C。
4. Isotopes | 同位素
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
同位素是同一元素的原子，具有相同的质子数但不同的中子数。这意味着它们具有相同的原子序数但不同的质量数。
For example, carbon-12 and carbon-14 are isotopes of carbon. Carbon-12 has 6 protons and 6 neutrons, whereas carbon-14 has 6 protons and 8 neutrons.
例如，碳-12和碳-14是碳的同位素。碳-12有6个质子和6个中子，而碳-14有6个质子和8个中子。
Isotopes of an element have the same chemical properties because they have the same number of electrons, but they may have different physical properties, such as density or rate of diffusion.
同位素的化学性质相同，因为它们具有相同的电子数，但它们的物理性质可能不同，例如密度或扩散速率。
Radioactive isotopes, such as carbon-14, are used in archaeology for dating ancient materials, while iodine-131 is used in medical imaging. These applications show the real-world significance of atomic structure.
放射性同位素，如碳-14，在考古学中用于测定古代材料的年代，而碘-131用于医学影像。这些应用展示了原子结构在现实世界中的重要性。
5. Electron Configuration | 电子排布
Electrons are arranged in shells (also called energy levels) around the nucleus. The first shell can hold up to 2 electrons, the second shell up to 8, and the third shell up to 8 (for the first 20 elements).
电子在原子核周围的壳层（也称为能级）中排列。第一个壳层最多可容纳2个电子，第二个壳层最多8个，
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