Foundations of Atomic and Subatomic Theory (Late 19th – Early 20th Century)

The turn of the 20th century marked one of the most transformative periods in the history of physics, as decades of philosophical speculation about the nature of matter gave way to rigorous experimental validation of atomic and subatomic structure. Prior to 1800, the concept of the atom was largely a metaphysical construct, with no direct empirical evidence to support its existence as a discrete, physical entity. The work of John Dalton in the early 19th century first codified atomic theory into a testable scientific framework, but it would take another 80 years of increasingly sophisticated experimentation to reveal that atoms themselves were composite objects, harboring a menagerie of smaller constituents. This chapter traces the stepwise development of atomic and subatomic theory from Dalton’s foundational postulates through the discovery of the electron, the establishment of the nuclear atomic model, and the identification of the proton and neutron, laying the experimental and conceptual groundwork for the emergence of particle physics as a distinct discipline. We will examine the design and logic of key experiments, the theoretical reasoning used to interpret their results, and the unresolved questions that would drive the next century of research into the fundamental structure of matter.

The Codification of Atomic Theory: Dalton’s Postulates

The collapse of the phlogiston theory in the late 18th century, following Antoine Lavoisier’s identification of oxygen as the agent of combustion, created a vacuum for a new theoretical framework to explain the quantitative regularities observed in chemical reactions. Two key experimental laws provided the foundation for this framework: the law of definite proportions, established by Joseph Proust in 1799, which showed that a given chemical compound always contains the same proportion of elements by mass; and the law of multiple proportions, observed by Dalton in 1803, which demonstrated that when two elements form more than one compound, the masses of one element that combine with a fixed mass of the other are in simple whole-number ratios. Dalton recognized that these regularities could be explained naturally if matter was composed of discrete, indivisible units—atoms—each with a characteristic mass for a given element.

Dalton’s atomic theory, first published in 1808, rested on four core postulates: 1. All matter is composed of tiny, indivisible particles called atoms. 2. All atoms of a given element are identical in mass and chemical properties; atoms of different elements differ in mass and properties. 3. Compounds are formed by the combination of atoms of different elements in simple, fixed whole-number ratios. 4. Chemical reactions involve the rearrangement of atoms, which are neither created nor destroyed in the process.

To test his theory, Dalton measured relative atomic masses for 21 elements using the law of multiple proportions. For example, carbon and oxygen form two common compounds: carbon monoxide (CO) and carbon dioxide (CO₂). In CO, 12 g of carbon combine with 16 g of oxygen, while in CO₂, 12 g of carbon combine with 32 g of oxygen. The ratio of oxygen masses combining with a fixed mass of carbon is 16:32 = 1:2, a simple whole number, which Dalton interpreted as evidence that CO contains one atom of carbon per atom of oxygen, while CO₂ contains one atom of carbon per two atoms of oxygen. This

Building on Dalton’s foundational postulates, 19th-century chemists and physicists gradually refined atomic theory through quantitative measurements of atomic mass and gas behavior, but the core assumption of indivisible, identical atoms remained unchallenged until the advent of precision electrical experimentation. The critical breakthrough came from studies of electrical discharges in rarefied gases, a line of inquiry that would ultimately reveal the first subatomic particle and prove atoms were composite structures. This section traces the stepwise evolution of experimental techniques and theoretical frameworks from Dalton’s chemical atom to the establishment of the nuclear atomic model, highlighting the logical reasoning used to interpret unexpected experimental results and the paradigm shifts that accompanied each discovery.

Refinements to Daltonian Atomic Theory: Avogadro’s Hypothesis and Atomic Mass Standardization

Dalton’s original atomic weight measurements suffered from a fundamental assumption error: he proposed that water was a binary compound with the formula HO, rather than the correct H₂O. This led him to assign oxygen an atomic mass of 7 (relative to hydrogen = 1), rather than the correct value of 16. For decades, inconsistent atomic weight measurements plagued chemistry, as researchers used different reference standards and assumed incorrect compound formulas. The resolution came in 1811, when Amedeo Avogadro proposed his famous hypothesis: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, regardless of the gas’s chemical identity[^1].

Avogadro’s hypothesis provided a direct link between measurable gas volumes and the number of atoms or molecules in a sample, allowing researchers to distinguish between atoms and molecules for the first time. For example, the observation that 2 volumes of hydrogen gas react with 1 volume of oxygen gas to form 2 volumes of water vapor could be explained if hydrogen and oxygen were diatomic molecules (H₂ and O₂), and water had the formula H₂O: 2 molecules of H₂ (4 hydrogen atoms) react with 1 molecule of O₂ (2 oxygen atoms) to form 2 molecules of H₂O (4 hydrogen atoms and 2 oxygen atoms), matching the 2:1 volume ratio. Despite its explanatory power, Avogadro’s hypothesis was largely ignored for 50 years, in part because Dalton and other leading chemists rejected the idea that identical atoms could combine to form molecules.

The turning point came at the 1860 Karlsruhe Conference, where Italian chemist Stanislao Cannizzaro presented a systematic method for calculating atomic masses using Avogadro’s hypothesis and the law of definite proportions. Cannizzaro’s method relied on measuring the densities of gaseous elements and compounds, then using Avogadro’s law to relate density ratios to molecular mass ratios. For example, the density of oxygen gas is 16 times that of hydrogen gas at the same temperature and pressure, so an oxygen molecule has 16 times the mass of a hydrogen molecule. If water is H₂O, then the mass of an oxygen atom is half the mass of an oxygen molecule, or 8 times the mass of a hydrogen atom? Wait no, hydrogen is H₂，所以氢原子质量是1，氢分子是2，氧分子是32，所以氧原子是16，对，这样就统一了原子量的标准，后来的门捷列夫元素周期表的成功也依赖于准确的原子量，所以Avogadro的假说其实是原子理论发展的关键一步，为后来的分子运动论，还有气体动理论打下了基础，气体动理论里可以算阿伏伽德罗常数，也就是1mol气体的分子数，大概是$6.022\times10^{23}$，这个数值后来也被用来算电子的电荷，比如密立根的油滴实验。

哦对，密立根的油滴实验是1909年做的，测电子的电荷，这个可以提一下，因为汤姆逊测了荷质比，但是不知道电荷是多少，密立根测了电荷，就可以算出电子的质量，这个很重要，所以可以在讲完汤姆逊发现电子之后，讲密立根的油滴实验，测电子的电荷，$e \approx 1.602\times10^{-19}\ \text{C}$，这样电子的质量就是$m_e = e/(e/m_e) \approx 9.11\times10^{-31}\ \text{kg}$，比氢原子的质量$1.67\times10^{-27}\ \text{kg}$小三个数量级，这个数据要准确。

然后接下来讲19世纪的真空技术和电学发展，为阴极射线实验提供了基础，1800年伏打发明伏打电池，提供了稳定的直流电源，1820年奥斯特发现电流的磁效应，说明电和磁是相互关联的，1831年法拉第发现电磁感应，1857年克鲁克斯发明了高真空泵，能制造出压强低到$10^{-3}\ \text{Pa}$的真空环境，这样放电管里的气体分子很少，就不会有电离，能观察到阴极射线的性质，这个要讲，因为实验技术的进步是科学发现的基础。

然后是阴极射线的研究，1858年普吕克尔在观察放电管里的放电现象的时候，发现阴极对面的玻璃管壁上有绿色的荧光，他意识到这是一种从阴极发出的射线，打到玻璃上产生的荧光，然后1869年希托夫发现这种射线是直线传播的，因为他在阴极和荧光屏之间放了一个障碍物，障碍物后面有阴影，1876年戈尔茨坦把这种射线叫做阴极射线，因为是从阴极发出的。

然后19世纪末的争论：阴极射线是粒子还是电磁波？支持电磁波的论据：1. 直线传播，和光一样；2. 能产生荧光；3. 速度接近光速。支持粒子的论据：1. 能产生热效应，说明有动能；2. 能推动小风轮，说明有动量；3. 能穿透薄金属箔，说明是小粒子。1895年勒纳德发现阴极射线能穿透薄铝箔，而且穿透能力和铝箔的厚度成指数关系，他认为是电磁波，因为光也能穿透，但是汤姆逊认为如果是电磁波的话，不会被磁场偏转，而1897年他做的实验证明阴极射线能被电场和磁场偏转，所以是带电粒子。

然后讲汤姆逊的实验，1897年，他用了不同的阴极材料（比如铂、铝、铁），不同的管内气体（比如氢气、氧气、二氧化碳），测出来的荷质比都是一样的，说明阴极射线是普遍的粒子，不是和特定元素相关的，然后他测了不同加速电压下的荷质比，发现荷质比和加速电压无关，说明电荷是固定的，然后他测了热效应，发现阴极射线打在温度计上，温度升高，说明有动能，然后他算出来电荷和氢离子的电荷差不多，所以质量很小，是比原子小的粒子，他当时叫corpuscle，后来才叫electron，这个是第一个被发现的基本粒子，推翻了原子不可分的假设，这个革命性的结论当时引起了很大的争议，比如开尔文勋