📚 Periodic Trends | 周期趋势
Periodic trends are repeating patterns in the properties of elements when they are arranged in order of increasing atomic number. Learning these trends helps us predict how an element will behave, even before we test it in the laboratory.
周期趋势是指元素按原子序数递增排列时，元素性质呈现出的重复规律。学习这些趋势可以帮助我们在实验室测试之前，就预测某种元素可能具有的行为。
1. The Periodic Table Layout | 元素周期表的结构
The periodic table arranges all known elements in rows called periods and columns called groups. Elements are placed in order of increasing atomic number, which equals the number of protons in the nucleus.
元素周期表将所有已知元素排列成横行（周期）和竖列（族）。元素按照原子序数递增排列，原子序数等于原子核中的质子数。
Each period starts when a new electron shell begins to fill. For example, sodium in period 3 has three electron shells, while lithium in period 2 has only two shells.
每当开始填充一个新的电子层，就开启一个新的周期。例如，第 3 周期的钠有三个电子层，而第 2 周期的锂只有两个电子层。
Groups are vertical columns. Elements in the same group have the same number of outer-shell electrons, so they show similar chemical reactions. This repeating pattern is the heart of periodic trends.
族是竖列。同一族元素具有相同的最外层电子数，因此它们表现出相似的化学反应。这种重复模式正是周期趋势的核心。
2. Electron Shells and Reactivity | 电子层与反应性
The first electron shell can hold up to 2 electrons. The second and third shells can each hold up to 8 electrons at KS3 level. We write electron arrangements such as 2,1 for lithium and 2,8,1 for sodium.
第一个电子层最多容纳 2 个电子。在 KS3 阶段，第二层和第三层各最多容纳 8 个电子。我们写出电子排布，例如锂为 2,1，钠为 2,8,1。
The number of outer-shell electrons controls how an element reacts. Metals have few outer electrons and tend to lose them, while non-metals have many outer electrons and tend to gain or share them.
最外层电子数决定着元素的反应方式。金属具有较少的最外层电子，倾向于失去电子；非金属具有较多的最外层电子，倾向于获得或共用电子。
A full outer shell makes an element very stable. This explains why the noble gases in group 0 are unreactive.
最外层为满电子层时，元素非常稳定。这解释了为什么第 0 族的惰性气体不活泼。
3. Metallic and Non-Metallic Character | 金属性与非金属性
Metals are found on the left and centre of the periodic table. Non-metals are found on the right. A zig-zag line separates them, with elements near the line often called semi-metals or metalloids.
金属位于元素周期表的左侧和中部。非金属位于右侧。一条锯齿形分界线将它们分开，靠近该线的元素通常称为半金属或类金属。
Across a period from left to right, metallic character decreases and non-metallic character increases. Down a group, metallic character generally increases.
在同一周期中从左到右，金属性减弱，非金属性增强。在同一族中从上到下，金属性通常增强。
For example, in period 3: sodium and magnesium are reactive metals, aluminium is a metal, silicon is a metalloid, and phosphorus, sulfur and chlorine are non-metals.
例如在第 3 周期中：钠和镁是活泼金属，铝是金属，硅是类金属，而磷、硫和氯是非金属。
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