📚 Organising the Elements | 元素周期表的编排
The periodic table is one of the most important tools in chemistry. It arranges all known elements in a logical order so that we can see patterns and predict how elements will behave. In KS3 chemistry, learning how elements are organised helps you understand why sodium reacts violently with water, why chlorine is toxic, and why helium hardly reacts at all.
周期表是化学中最重要的工具之一。它把所有已知元素按逻辑顺序排列，使我们能够看出规律并预测元素的性质。在 KS3 化学中，学习元素如何编排可以帮助你理解为什么钠与水剧烈反应、为什么氯有毒、以及为什么氦几乎不发生反应。
1. What Are Elements? | 什么是元素？
An element is a pure substance made of only one type of atom. Each element has a different number of protons in its atoms, which gives it a unique identity. There are about 100 naturally occurring and synthetic elements known today.
元素是由同一种原子组成的纯净物。每种元素原子中的质子数不同，这赋予了它独特的身份。目前已知大约有100种天然存在和人工合成的元素。
Atoms of the same element all have the same number of protons. For example, every carbon atom has six protons, so no other element can be carbon.
同一种元素的原子都具有相同的质子数。例如，每个碳原子都有六个质子，因此其他任何元素都不可能是碳。
2. Chemical Symbols | 化学符号
Each element has a chemical symbol made of one or two letters. The first letter is always a capital letter, and the second letter, if there is one, is always lower case. Some symbols come from Latin or Greek names, such as Na for sodium (natrium) and Fe for iron (ferrum).
每种元素都有一个由一两个字母组成的化学符号。第一个字母总是大写，第二个字母（如果有）总是小写。有些符号来自拉丁语或希腊语名称，例如 Na 代表钠（natrium），Fe 代表铁（ferrum）。
Writing symbols correctly is important because one small mistake can change the element. For example, Co is cobalt, but CO is a compound made of carbon and oxygen.
正确书写符号很重要，因为一个小的错误就可能改变元素。例如，Co 是钴，而 CO 是由碳和氧组成的化合物。
| Element | Symbol | Note |
| Hydrogen | H | Simplest element |
| Carbon | C | Found in all living things |
| Sodium | Na | From Latin natrium |
| Iron | Fe | From Latin ferrum |
| Lead | Pb | From Latin plumbum |
3. Why Organise Elements? | 为什么要编排元素？
With so many elements, it is useful to group those that are similar. Organising elements lets chemists predict how an element will behave without testing every single one. It also shows patterns, such as the gradual change from metals to non-metals.
元素种类繁多，将相似的元素归类十分有用。编排元素使化学家无需逐一实验就能预测元素的性质。它还展示了一些规律，例如从金属到非金属的逐渐变化。
The periodic table is sometimes called the chemist's map because it shows relationships between elements at a glance. Once you know the position of an element, you can often predict its physical and chemical properties.
周期表有时被称为化学家的地图，因为它可以让你一眼看出元素之间的关系。一旦知道元素在周期表中的位置，你通常就能预测它的物理性质和化学性质。
4. Early Ideas: Döbereiner and Newlands | 早期尝试：德贝赖纳与纽兰兹
Before Mendeleev, chemists noticed that some elements could be placed into small sets of three with similar properties. Döbereiner called these sets 'triads'. Later, John Newlands arranged elements by increasing atomic mass and noticed that every eighth element seemed to have similar properties, which he called the 'law of octaves'.
在门捷列夫之前，化学家注意到一些元素可以组成性质相似的三个一组。德贝赖纳称这些组合为“三元组”。后来，约翰·纽兰兹按原子质量递增的顺序排列元素，发现每隔八个元素似乎具有相似性质，他称之为“八音律”。
These early ideas were not perfect, but they helped later scientists build the modern table. They showed that element properties repeat in a periodic pattern when elements are placed in order.
这些早期的想法并不完美，但它们帮助后来的科学家建立了现代周期表。它们表明，当元素按顺序排列时，元素性质会呈现周期性的重复规律。
5. Mendeleev's Periodic Table | 门捷列夫周期表
In 1869, Dmitri Mendeleev created a table by placing elements in order of increasing atomic mass, but he also grouped elements with similar properties into columns. To make the patterns work, he left gaps for elements that had not yet been discovered and predicted their properties with impressive accuracy.
1869年，德米特里·门捷列夫创建了一张表格，将元素按原子质量递增的顺序排列，但同时把性质相似的元素分在同一列。为了使规律成立，他为尚未发现的元素留下空位，并惊人准确地预测了它们的性质。
For example, Mendeleev predicted the existence of an element he called eka-aluminium. When gallium was discovered, its properties matched many of his predictions, which convinced other scientists that his table was useful.
例如，门捷列夫预测了一种他称之为“类铝”的元素的存在。当镓被发现时，它的性质与门捷列夫的许多预测相符，这使其他科学家相信他的周期表是有效的。
6. The Modern Periodic Table | 现代周期表
The modern periodic table arranges elements in order of increasing atomic number, which is the number of protons in an atom. This arrangement clears up the few cases where ordering by atomic mass gave the wrong pattern, such as iodine and tellurium.
现代周期表按原子序数（原子中的质子数）递增的顺序排列元素。这种排法解决了少数按原子质量排列会打乱规律的情况，例如碘和碲的位置。
Relative atomic mass is also shown on most periodic tables. It is an average mass that takes account of the different isotopes of an element. However, the order of elements is based on atomic number, not atomic mass.
大多数周期表上还标有相对原子质量。它考虑了元素不同同位素后的平均质量。然而，元素的排列顺序依据的是原子序数，而不是原子质量。
Atomic number = number of protons
7. Groups and Periods | 族与周期
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