Masses of Atoms and Molecules | 原子与分子的质量

📚 Masses of Atoms and Molecules | 原子与分子的质量

Understanding the masses of atoms and molecules is central to quantitative chemistry. Cambridge A Level Chemistry expects you to use relative masses, interpret mass spectra and carry out mole calculations confidently.

理解原子和分子的质量是定量化学的核心。剑桥A Level化学要求你熟练掌握相对质量、解读质谱图并进行摩尔计算。

This topic links the submicroscopic scale of particles to the laboratory masses measured in grams, so it appears in stoichiometry, titration, gas volume and formula determination.

本主题将粒子的亚微观尺度与实验中以克计量的质量联系起来，因此它出现在化学计量、滴定、气体体积和化学式确定等内容中。

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1. The Atomic Mass Unit | 原子质量单位

A single carbon-12 atom has a mass of about 1.99265 × 10⁻²⁶ kg, which is far too small for practical calculations.

一个碳-12原子的实际质量约为 1.99265 × 10⁻²⁶ kg，这个数值太小，不便于实际计算。

Chemists therefore define the unified atomic mass unit, symbol u, as exactly 1/12 of the mass of one neutral ¹²C atom. One unit equals 1.66054 × 10⁻²⁷ kg.

因此化学家定义统一原子质量单位，符号为 u，恰好等于一个中性 ¹²C 原子质量的1/12。1 u = 1.66054 × 10⁻²⁷ kg。

The hydrogen-1 atom has a mass close to 1.007 u, while an oxygen-16 atom has a mass close to 15.995 u.

氢-1原子的质量接近1.007 u，而氧-16原子的质量接近15.995 u。

All relative atomic and molecular masses are ratios, so they have no units and do not depend on the current definition of the kilogram.

所有相对原子质量和相对分子质量都是比值，因此没有单位，也不依赖于千克的当前定义。

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2. Relative Atomic Mass, Aᵣ | 相对原子质量 Aᵣ

Relative atomic mass, Aᵣ, is the weighted mean mass of all the naturally occurring isotopes of an element relative to 1/12 of the mass of a carbon-12 atom.

相对原子质量 Aᵣ 是某元素所有天然同位素原子质量的加权平均值，相对于碳-12原子质量的1/12。

Because Aᵣ is a weighted average, it is not usually a whole number. The weighting depends on the natural abundance of each isotope.

由于 Aᵣ 是加权平均值，通常不是整数。权重取决于每种同位素的天然丰度。

For example, chlorine has Aᵣ ≈ 35.5 because natural chlorine contains both ³⁵Cl and ³⁷Cl isotopes in significant amounts.

例如，氯的 Aᵣ ≈ 35.5，因为天然氯同时含有大量的 ³⁵Cl 和 ³⁷Cl 同位素。

Carbon-12 is assigned a relative isotopic mass of exactly 12.000, but the natural element carbon has Aᵣ ≈ 12.011 because of the presence of ¹³C and a trace of ¹⁴C. In simple A Level calculations, carbon is usually taken as 12.0.

碳-12的相对同位素质量被指定为12.000，但天然碳元素的 Aᵣ ≈ 12.011，因为存在 ¹³C 和微量 ¹⁴C。在简单的A Level计算中，碳通常取12.0。

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3. Mass Spectrometry and Relative Isotopic Mass | 质谱法与相对同位素质量

A mass spectrometer separates gaseous positive ions by their mass-to-charge ratio, written m/z, and produces a spectrum of peaks.

质谱仪根据质荷比（写作 m/z）分离气态正离子，并产生质谱峰。

Relative isotopic mass is the mass of one atom of a particular isotope relative to 1/12 of the mass of a carbon-12 atom. For example, ³⁵Cl has a relative isotopic mass of 35.0 and ³⁷Cl has 37.0.

相对同位素质量是某一特定同位素的一个原子质量相对于碳-12原子质量的1/12。例如 ³⁵Cl 的相对同位素质量为35.0，³⁷Cl 为37.0。

In a mass spectrum of a molecular substance, the peak at the highest m/z is often the molecular ion peak, M⁺. This peak gives the relative molecular mass if fragmentation is not significant.

在分子物质的质谱图中，最高 m/z 处的峰通常是分子离子峰 M⁺。如果碎片化不明显，该峰给出相对分子质量。

The main stages of a mass spectrometer are vaporisation, ionisation, acceleration, deflection and detection. Electron impact ionisation removes one electron to form a positive ion, usually M⁺ from a molecule.

质谱仪的主要阶段是汽化、电离、加速、偏转和检测。电子轰击电离会移去一个电子形成正离子，通常由分子形成 M⁺。

A lighter ion is deflected more than a heavier ion with the same charge. A more highly charged ion is also deflected more in the magnetic field.

带相同电荷时，较轻的离子比较重的离子偏转更多。带电荷越多的离子在磁场中偏转也越多。

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4. Calculating Aᵣ from Isotopic Abundances | 由同位素丰度计算 Aᵣ

When isotopic masses and percentage abundances are known, Aᵣ is calculated using the weighted average expression:

当已知同位素质量和百分丰度时，Aᵣ 通过加权平均表达式计算：

Aᵣ = Σ (isotopic mass × percentage abundance) ÷ 100

Example: chlorine has ³⁵Cl with mass

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