📚 The Periodic Table | 元素周期表
The periodic table is one of the most important tools in science. It organises every known element by increasing atomic number and reveals repeating patterns in physical and chemical properties.
元素周期表是科学中最重要的工具之一。它按原子序数递增的顺序排列所有已知元素，并展现出物理和化学性质中的重复规律。
1. What Is the Periodic Table? | 什么是元素周期表？
The periodic table is a tabular arrangement of the known elements. Each element is placed in a specific cell that shows its symbol, atomic number, and relative atomic mass.
元素周期表是已知元素的表格排列方式。每种元素都被放在特定的单元格中，显示其符号、原子序数和相对原子质量。
The table is organised into horizontal rows called periods and vertical columns called groups. Elements in the same group usually have the same number of outer-shell electrons, so they tend to behave in similar ways.
该表被组织成横向的周期和纵向的族。同一族中的元素通常具有相同的最外层电子数，因此它们往往表现出相似的行为。
2. Structure of an Atom | 原子的结构
An atom consists of a tiny, dense nucleus made up of protons and neutrons. Negatively charged electrons move around the nucleus in energy levels, also called electron shells.
原子由一个微小而致密的原子核组成，原子核中包含质子和中子。带负电荷的电子在原子核周围的能量层（即电子壳层）中运动。
The atomic number (Z) is the number of protons in the nucleus. In a neutral atom, the number of electrons equals the number of protons. The mass number (A) is the total number of protons plus neutrons.
原子序数（Z）是原子核中的质子数。在电中性的原子中，电子数等于质子数。质量数（A）是质子数加中子数的总和。
Number of protons = Number of electrons (in an atom)
在原子中，质子数等于电子数。
Mass number = Protons + Neutrons
质量数等于质子数加中子数。
3. How Are Elements Arranged? | 元素是如何排列的？
Elements in the modern periodic table are arranged in order of increasing atomic number. This ordering ensures that elements with similar electron configurations fall into the same vertical column.
现代元素周期表中的元素按原子序数递增的顺序排列。这种排列方式确保具有相似电子排布的元素处于同一纵列。
Elements in the same period have the same number of occupied electron shells. For example, period 2 elements all have two shells, while period 3 elements all have three shells.
同一周期的元素具有相同的电子壳层数。例如，第二周期的元素都有两个壳层，第三周期的元素都有三个壳层。
Elements in the same group have the same number of outer-shell electrons. Group 1 elements have one outer electron, group 2 have two, and group 7 have seven.
同一族的元素具有相同的最外层电子数。第1族元素有一个外层电子，第2族有两个，第7族有七个。
| Element | Symbol | Atomic Number | Group | Period |
|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 |
| Helium | He | 2 | 0 (18) | 1 |
| Lithium | Li | 3 | 1 | 2 |
| Carbon | C | 6 | 4 | 2 |
| Oxygen | O | 8 | 6 | 2 |
| Sodium | Na | 11 | 1 | 3 |
The table above shows a few elements and their positions. Notice that elements in the same group count their outer electrons in the same way.
上表显示了一些元素及其位置。注意同一族的元素以相同的方式计算其外层电子数。
4. Metals vs Non-Metals | 金属与非金属
The periodic table can be roughly divided into metals, found on the left and in the middle, and non-metals, found on the upper right. A zig-zag step line separates the two regions.
元素周期表可以大致分为金属和非金属：金属位于左侧和中部，非金属位于右上方。一条锯齿状阶梯线将两个区域分开。
Metals are usually shiny, malleable, and good conductors of heat and electricity. When they react, they often lose outer electrons and become positive ions.
金属通常有光泽、延展性好，并且是良好的热和电导体。它们在反应时往往会失去外层电子，变成正离子。
Non-metals are often dull and poor conductors, and when they react they tend to gain electrons or share them to form negative ions or covalent compounds.
非金属通常暗淡且导电性差，在反应时倾向于得到电子或共享电子，从而形成负离子或共价化合物。
- Examples of metals: sodium, magnesium, iron, copper, zinc.
- 金属的例子：钠、镁、铁、铜、锌。
- Examples of non-metals: carbon, nitrogen, oxygen, chlorine, sulfur.
- 非金属的例子：碳、氮、氧、氯、硫。
5. Group 1: Alkali Metals | 第1族：碱金属
Group 1 elements are known as alkali metals. They include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and caesium (Cs). These metals all have one electron in their outer shell.
第1族元素被称为碱金属，包括锂（Li）、钠（Na）、钾（K）、铷（Rb）和铯（Cs）。这些金属的最外层都有一个电子。
Alkali metals are soft, have low densities, and are extremely reactive. Their reactivity increases as you go down the group, because the outer electron is further from the nucleus and more easily lost.
碱金属质地软、密度小，且非常活泼。其活动性沿族向下增强，因为外层电子离核更远，更容易失去。
They all react vigorously with cold water to produce a metal hydroxide and hydrogen gas. For example, sodium reacts as follows:
它们都能与冷水剧烈反应，生成金属氢氧化物和氢气。例如，钠的反应如下：
2Na + 2H₂O → 2NaOH + H₂
This reaction is exothermic, and with potassium or caesium it can burst into flame.
该反应放热，如果是钾或铯，甚至会燃烧起来。
6. Group 7: Halogens | 第7族：卤素
Group 7 elements are called halogens. They include fluorine (F), chlorine (Cl), bromine (Br), and iodine (I). They all have seven outer electrons and exist as diatomic molecules, such as F₂, Cl₂, Br₂, and I₂.
第7族元素被称为卤素，包括氟（F）、氯（Cl）、溴（Br）和碘（I）。它们都有七个外层电子，并以双原子分子形式存在，如 F₂、Cl₂、Br₂ 和
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