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Of all chemical theories, the most fundamental and important one is the atomic theory. The concept of the atom has a long history. The ancient Greek philosopher Democritus first proposed the theory of atoms before 450 BC; he believed that all things in the universe are composed of the smallest, hardest, indestructible, and indivisible particles of matter. Such particles are called atoms. The great diversity among all things in the universe is due to the differences in the number, shape, and arrangement of the atoms that make them up. This is a simple materialistic view of things. During the same period in China’s Spring and Autumn and Warring States eras, some renowned thinkers such as Hui Shi and Mozi also put forward, from different perspectives, the idea that matter has an indivisible smallest unit. As Mozi said, “It will not move unless it is at half; the principle lies in the midpoint.” ”In other words, once the material is reduced to less than half of its original amount, it can no longer be further cut. This notion of an \"endpoint\" or something that cannot be further divided corresponds to the ancient concept of atoms; however, at that time it was mainly an intuitive idea stemming from naive materialism among philosophers, nothing more than a philosophical speculation. In 1805, the British chemist Dalton proposed a hypothesis: all matter is composed of several different types of particles, corresponding to different elements. He called these particles atoms, a term derived from the Greek word atomos, meaning indivisible. Dalton’s atomic theory successfully explained the quantitative relationships between various substances during chemical reactions—such as the law of conservation of mass, the law of definite proportions, the law of multiple proportions, and the law of equivalents. And it was confirmed by further research in chemistry and physics, thus this hypothesis became the atomic theory. Dalton’s atomic theory grasped the core and most fundamental aspects of chemistry; it allowed for the explanation of various chemical phenomena and the inherent relationships between chemical laws through the combination and separation of atoms. Through continuous refinement, this theory eventually became a unified theory for explaining chemical phenomena, holding great scientific and philosophical significance. Engels highly praised Dalton’s atomic theory, stating that it was a discovery that could provide a foundation for all of science and lay a solid basis for research work. He also pointed out that \"a new era in chemistry began with the atomic theory; therefore, the father of modern chemistry is not Lavoisier but Dalton.\" In 1897, the electron was discovered. Based on the understanding that all atoms contain electrons, W. Thomson proposed in 1898 an atomic model in which electrons are immersed within a \"uniformly distributed positively charged sphere\". This is Thomson’s atomic “immersion model”. This is a rejection of the idea that “atoms are solid spheres” and that “atoms cannot be divided any further”; it represents a modification to Dalton’s atomic model. In 1911, E. Rutherford discovered through alpha particle scattering experiments that the positive charge in an atom is not “uniformly distributed”. He proposed that the positive charge in an atom is concentrated in a very small, compact region called the nucleus. The number of electrons moving at high speed around it is equal to the positive charge of the nucleus; this is Rutherford’s nuclear model of the atom. This is the second major breakthrough in the concept of atoms. In 1923, Rutherford’s student N. Bohr, building on his teacher’s model of the atomic nucleus as well as Planck’s theory of quantum mechanics, introduced the concept of quantization conditions. This allowed him to successfully explain the patterns in the hydrogen atom spectrum, leading to the proposal of the \"planetary atom model,\" in which electrons move around the nucleus along fixed orbits. But this is still the content of the old quantum theory. In 1926, E. Schrödinger of Austria proposed the atomic concept of wave mechanics or quantum mechanics, building on the wave-particle duality of electron motion outside the nucleus and integrating Bohr’s idea of fixed orbits, Heisenberg’s uncertainty principle, and de Broglie’s insight into the wave nature of electrons. According to his atomic wave mechanics model, electrons are three-dimensional waves surrounding the atomic nucleus; they are probability waves that obey the laws of quantum mechanics. This represents a new quantum theory of the motion of electrons outside the nucleus, and it forms the basis of the modern concept of the atom. In short, an atom is the smallest unit of a element, and it is the smallest particle that constitutes the \"molecules\" of elements and compounds. Atoms of different elements have different structures and masses. Atoms of the same element form simple substances, while atoms of different elements form compounds. In ordinary chemical reactions, the atomic nuclei do not change; only the valence electrons outside the nucleus are altered. Atoms are the basic particles through which matter undergoes chemical reactions. In nuclear reactions, atomic nuclei change. Atoms are composed of even smaller particles (electrons, neutrons, protons, etc.).