Periodicity: Trends in Period 3 Elements | 周期性：第3周期元素性质递变规律

📚 Periodicity: Trends in Period 3 Elements | 周期性：第3周期元素性质递变规律

Periodicity is the repeating pattern of physical and chemical properties across a period of the periodic table. In Cambridge A-Level Chemistry, Period 3 (Na to Ar) is used to illustrate how atomic structure drives trends in radius, ionisation energy, electronegativity, melting point, and oxide/chloride behaviour.

周期性是指元素物理和化学性质在周期表中沿周期呈现的重复变化规律。剑桥A-Level化学以第3周期（钠到氩）为例，说明原子结构如何决定原子半径、电离能、电负性、熔点以及氧化物和氯化物性质的递变。

1. The Meaning of Periodicity | 周期性的含义

Periodicity means that elements in the same period show a gradual change from metallic to non-metallic character. Across Period 3, from sodium to argon, the same valence shell is being filled, but nuclear charge increases steadily. This causes a clear change in structure, bonding and reactivity.

周期性是指同一周期元素的性质从金属性逐渐过渡到非金属性。第3周期从钠到氩，电子填入同一最外层，但核电荷逐步增加，因此元素的结构、成键方式和反应性发生明显变化。

• Across a period, the number of inner shells remains constant, so shielding is approximately constant.
• Across a period, effective nuclear charge increases because the number of protons increases.
• These two factors control most periodicity trends: atomic radius, ionisation energy, electronegativity and structure.

• 同一周期内，内层电子数不变，屏蔽效应大致相同。
• 同一周期内，质子数增加，有效核电荷增大。
• 这两个因素决定了原子半径、电离能、电负性以及结构等大多数周期性递变规律。

2. Atomic Radius Across Period 3 | 第3周期原子半径的递变

Atomic radius decreases across Period 3 from sodium to chlorine. All electrons are added to the same n = 3 shell, but the nuclear charge increases from +11 to +17. The outer electrons experience a greater effective nuclear charge, pulling the electron cloud closer to the nucleus.

第3周期原子半径从钠到氯逐渐减小。所有电子都填入 n = 3 的同一电子层，但核电荷从 +11 增大到 +17。外层电子受到的有效核电荷增大，电子云被更紧密地拉向原子核。

Argon is usually excluded from this trend because it exists as isolated atoms and does not form a covalent bond. Its van der Waals radius is larger than the covalent radii of Na to Cl.

氩通常不参与该趋势比较，因为它以孤立原子存在，不形成共价键。其范德华半径大于钠到氯的共价半径。

3. First Ionisation Energy Across Period 3 | 第3周期第一电离能的递变

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. The general trend across Period 3 is an increase, but there are two important drops: between Mg and Al, and between P and S.

第一电离能是指从1摩尔气态原子中移走1摩尔电子形成1摩尔气态1+离子所需的能量。第3周期第一电离能总体呈增大趋势，但在镁到铝、磷到硫之间出现两次明显下降。

X(g) → X⁺(g) + e⁻