Periodicity of Chemical Properties | 化学性质的周期性

📚 Periodicity of Chemical Properties | 化学性质的周期性

The periodic table arranges elements in order of increasing atomic number, revealing a recurring pattern in chemical and physical properties known as periodicity. This pattern arises because elements with the same number of outer-shell electrons exhibit similar chemical behaviour. In A-Level chemistry, periodicity is best illustrated across Period 3 (Na to Ar) and down groups such as Group 2 and Group 17. Key properties that show periodic trends include atomic radius, ionisation energy, electronegativity, melting point, and the acid-base character of oxides and chlorides.

元素周期表按原子序数递增顺序排列，揭示了化学和物理性质周期性重复出现的规律。这种规律源于具有相同最外层电子数的元素表现出相似的化学行为。在A-Level化学中，周期性在第三周期（钠到氩）以及第二族、第十七族等族中表现得尤为明显。呈现周期性变化的重要性质包括原子半径、电离能、电负性、熔点以及氧化物和氯化物的酸碱性等。

1. What is Periodicity? | 什么是周期性？

Periodicity is the tendency of element properties to recur at regular intervals when elements are arranged by atomic number. The modern explanation lies in the periodic repetition of electron configurations. For example, both lithium (1s²2s¹) and sodium (1s²2s²2p⁶3s¹) have a single outer s electron, giving them similar reactivity. Moving across a period, the outer electron shell fills progressively, leading to gradual changes from metallic to non-metallic character. The periodic law states that the properties of elements are a periodic function of their atomic numbers.

周期性是指元素按原子序数排列时，其性质每隔一定数目出现相似的规律。现代的解释基于电子构型的周期性重复。例如，锂（1s²2s¹）和钠（1s²2s²2p⁶3s¹）最外层都有一个s电子，因此具有相似的反应性。沿周期从左到右，外层电子逐步填充，导致从金属性到非金属性的渐变。周期律指出，元素的性质是其原子序数的周期函数。

2. Atomic Radius across a Period | 同周期原子半径的变化

Atomic radius decreases across a period from left to right. In Period 3, the covalent radius falls from 186 pm for Na to 99 pm for Ar. This decrease occurs because the number of protons (nuclear charge) increases, pulling the electron cloud closer to the nucleus. Although electrons are being added to the same principal quantum shell (n=3), the increase in shielding is minor compared with the increase in nuclear attraction. Hence, the effective nuclear charge, Zeff, experienced by the outer electrons rises, contracting the atom.

同一周期从左到右，原子半径减小。第三周期中，共价半径从钠的186 pm下降到氩的99 pm。这是由于质子数（核电荷）增加，将电子云拉向原子核。尽管电子被加到同一主量子层（n=3），但屏蔽作用的增加与核引力增加相比很小。因此，外层电子感受到的有效核电荷（Zeff）增大，使原子收缩。

3. Ionic Radius Trends | 离子半径趋势

Ionic radius depends on the number of protons and the number of electrons. For cations, the loss of outer electrons reduces electron–electron repulsion and often removes the outermost shell entirely, making the ion much smaller than the parent atom. For example, Na⁺ (radius 102 pm) is smaller than Na (186 pm). For anions, the addition of electrons increases repulsion, expanding the electron cloud; Cl⁻ (181 pm) is larger than Cl (99 pm). Across Period 3, the ionic radii of positive ions decrease (Na⁺ > Mg²⁺ > Al³⁺) due to increasing nuclear charge, while the negative ions also decrease (P³⁻ > S²⁻ > Cl⁻) for the same reason.

离子半径取决于质子数和电子数。对于阳离子，失去外层电子减少了电子-电子排斥，并且往往完全移除了最外电子层，因此离子比母原子小得多。例如Na⁺（半径102 pm）小于Na（186 pm）。对于阴离子，电子增加使排斥增强，电子云膨胀；Cl⁻（181 pm）大于Cl（99 pm）。沿第三周期，阳离子半径依次减小（Na⁺ > Mg²⁺ > Al³⁺），因为核电荷递增；阴离子半径也依次减小（P³⁻ > S²⁻ > Cl⁻），原因相同。

4. First Ionisation Energy | 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. Across a period, first ionisation energy generally increases because the atomic radius decreases and the effective nuclear charge increases, making the outer electrons harder to remove

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

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