Thread Content
Molecules are the most fundamental concept in chemistry. A molecule is a system composed of atoms. More than 100 types of atomic elements have been discovered to date. All physical substances in the world are composed of molecules, and there are a vast number of different types of molecules; over ten million stable molecules are known so far. Chemistry is a science that studies molecules; the task of chemists is to analyze and understand the composition and structure of molecules, as well as to identify the underlying patterns, so as to develop methods for synthesizing various molecules with the properties desired by humanity, in order to meet its growing needs. The molecule is a fundamental chemical concept that is evolving. As early as 1811, Aavogadro of Italy proposed the hypothesis of molecules in order to explain the experimental finding by the French scientist Gay-Lussac that \"gases combine in simple integer ratios in terms of volume.\" He pointed out that whether it is an element or a compound, there is a molecular stage in the process of continuous division. A molecule is the smallest unit that constitutes a substance and possesses certain properties; it is a particle more complex than an atom, and molecules are composed of atoms. This bold hypothesis successfully explained Gay-Lussac’s law of the combined volumes of gases and resolved the doubts of Dalton and others. However, due to opposition from certain prevailing views at the time, it was not recognized by contemporaries. It was not until 50 years later, in 1860, during an international chemistry conference held in Karlsruhe, that another Italian chemist, Cannizzaro, clarified it, thereby giving rise to the atomic-molecular theory. The clarification of molecular concepts promoted the development of chemical science, and soon important concepts and theories related to molecules were established, such as chemical bonds, molecular structure, and the three-dimensional shape of molecules. Starting from the 1930s, new experimental techniques such as spectroscopy and diffraction were employed to determine the structure of molecules, and the newly emerging quantum mechanics quickly became a theoretical tool for studying the behavior of electrons in molecules. Photos of molecular images captured by electron microscopy prove the actual existence of molecules. Not only are solids, liquids, and gases composed of molecules, but there are also polymers or macromolecules. In 1927, Staudinger in Germany proposed the theory of macromolecules, which gained widespread recognition after the 1940s; many substances such as diamond, quartz, silicates, ionic salts, transition metal oxides, and sulfides were discovered and classified as “macromolecules” or “giant molecules”. Over the past decade or so, experimental methods for studying short-lived molecules have increased, enabling a dynamic view of molecules and clarifying that molecules are not \"rigid\" and that their stability is relative. Species, transitional intermediates, or molecules that were previously unknown and thought to be impossible to exist are now not only known to exist, but their structures and properties have also been understood. In particular, the advancements in radio astronomy and molecular spectroscopy have provided people with more knowledge related to \"unstable molecules\". In addition, many unknown molecular compounds have been discovered, such as inclusion complexes, channel inclusion complexes, cyclic ether and pocket ether host-guest complexes, intercalates, nucleic acid-protein complexes, etc. These can insert (or contain) other molecules within ordered molecular systems; in other words, they are molecular assemblies that are new \"molecules\" with special properties. After the 1960s, experimental instruments and methods for probing molecular structures developed at a rapid pace. In particular, the progress in spectroscopy and the successes in laser research led to the realization that molecules needed to be studied in depth as a distinct science, giving rise to molecular science. Molecular science, in a broad sense, is the science that studies natural phenomena at the molecular level. It includes certain aspects of zoology, botany, genetics, physiology, the theory of physical properties, and planetary and earth sciences; most of molecular biology, as well as all of chemistry and its related disciplines. In a narrow sense, molecular science aims to gain a thorough and precise understanding of the nature of various different types of molecules, to explore their new properties and functions, and to study them from a structural perspective. Secondly, it involves designing molecules with desired properties (molecular design), as well as devising the reaction pathways necessary for synthesizing new molecules (reaction design). Thirdly, it is to propose new theories and new experimental methods to achieve these goals. For example, there are interdisciplinary fields such as “molecular engineering” and “molecular technology”. It now seems that the fundamental content of the molecular concept should reflect the important understanding of \"the molecular nature of matter\", that is, molecules represent a level of division of matter; they are the basic units that make up this level, and these units or their collections should embody all the properties of matter itself. Of course, the size of such units is relative; substances in the gaseous state are almost entirely composed of small molecules. However, in solid or liquid states, a substance can be composed of small molecules, or it can be composed of macromolecules or giant molecules. The concept of molecule has **significantly evolved** in modern times, encompassing many new aspects, which is an inevitable outcome of the development of modern chemistry. There have also been new understandings of molecules: for example, \"a stable smallest entity formed by the combination of one or more atoms is generally referred to as a molecule; molecules are the basic units that participate in chemical reactions\"; \"a molecule is a group of atoms that are tightly bound together\"; \"molecules are units formed by the combination of atoms, and such molecular units can be very small, such as hydrogen molecules, or very large, such as crystalline table salt\", etc.