📚 Describe the Trends in Physical Properties of Period 3 Elements | 描述第三周期元素物理性质的变化趋势
Across Period 3 from sodium to argon, the physical properties of elements change in a largely predictable way because the number of protons increases while the 3s and 3p subshells are being filled. This article describes and explains the trends in atomic radius, first ionisation energy, electronegativity, melting point and electrical conductivity.
在第三周期中，从钠到氩，随着核电荷数增加、3s 和 3p 亚层逐步填充，元素的物理性质呈现出有规律的变化。本文将描述并解释原子半径、第一电离能、电负性、熔点以及导电性的变化趋势。
1. Electron Configurations and Periodicity Overview | 电子排布与周期性概述
Across this period, electrons are added to the same outer shell: the 3s subshell is filled first, followed by the 3p subshell. Because the inner electron shells remain unchanged, the shielding of the outer electrons by inner electrons stays approximately constant.
第三周期包括从钠（Na，Z = 11）到氩（Ar，Z = 18）的元素。在这一周期中，电子填入相同的外层：首先填充 3s 亚层，然后填充 3p 亚层。由于内层电子数保持不变，内层电子对外层电子的屏蔽作用基本不变。
The electron configurations are: Na [Ne]3s¹, Mg [Ne]3s², Al [Ne]3s²3p¹, Si [Ne]3s²3p², P [Ne]3s²3p³, S [Ne]3s²3p⁴, Cl [Ne]3s²3p⁵, Ar [Ne]3s²3p⁶.
电子排布式依次为：Na [Ne]3s¹、Mg [Ne]3s²、Al [Ne]3s²3p¹、Si [Ne]3s²3p²、P [Ne]3s²3p³、S [Ne]3s²3p⁴、Cl [Ne]3s²3p⁵、Ar [Ne]3s²3p⁶。
This regular filling pattern produces the periodic trends in physical properties because the outer electrons experience an increasing effective nuclear charge while the shielding effect remains nearly constant.
这种规则的填充方式导致物理性质呈周期性变化，因为外层电子感受到的有效核电荷逐渐增大，而屏蔽效应基本保持不变。
2. Atomic Radius Across Period 3 | 第三周期的原子半径
Atomic radius is measured as half the distance between the nuclei of two bonded atoms of the same element. Going from sodium to chlorine, the atomic radius decreases steadily.
原子半径定义为同种元素两个成键原子核间距离的一半。从钠到氯，原子半径逐渐减小。
This happens because the number of protons in the nucleus increases across the period, so the nuclear charge becomes larger. However, the added electrons enter the same outer shell and the shielding by inner shells remains almost constant. As a result, the effective nuclear charge felt by the outer electrons increases, pulling the outer shell closer to the nucleus.
这是因为周期中核内质子数增加，核电荷增大；但新增加的电子进入同一外层，内层屏蔽作用基本不变，因此外层电子感受到的有效核电荷增加，使外层电子被更强烈地拉向原子核。
The general trend can be remembered as: more protons, same shielding, smaller atom.
这一趋势可以概括为：质子数增加、屏蔽作用不变、原子半径减小。
3. First Ionisation Energy Across Period 3 | 第三周期的第一电离能
The first ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. For a general element X, this can be written as:
第一电离能是指从 1 mol 气态原子中移除 1 mol 电子、形成 1 mol 气态 +1 离子所需的能量。对一般元素 X，可表示为：
X(g) → X⁺(g) + e⁻
The first ionisation energy generally increases from sodium to argon. This is because the nuclear charge increases, the atomic radius decreases, and the outer electrons are held more tightly.
从钠到氩，第一电离能总体呈增大趋势。原因是核电荷
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