After determining the experimental formula and relative molecular weight, the understanding of an organic molecule has not been resolved because often several organic compounds have the same molecular formula, but their physical and chemical properties are not the same. In 1822, Weiler and Li Bixi respectively discovered silver isocyanate and silver fulminate, which were analyzed to be composed of one atom each of Ag, N, C, and O, but with completely different physical and chemical properties. Later, after careful research, Bai Zeli proved that this phenomenon did indeed exist. He referred to this phenomenon of having the same molecular formula but different structures as isomerism (abbreviated as isomerism). Two or more substances with the same composition are called isomers. He also explained that the differences in isomers are due to the different ways in which each atom in the molecule binds, and this different binding is called structure. Since the discovery of this phenomenon, organic chemistry has faced a problem of how to determine these structures. Without a solution, it cannot be considered a science. Later, through continuous exploration and thinking, the correct concept of structure was gradually established.

1. Two important fundamental rules of Kekul é (A.) and Couper (A.) (1857)
(1) Carbon atoms are tetravalent: whether in simple or complex compounds, the number of carbon atoms and other atoms always maintains a certain proportion. For example, CH4, CHCl3, CO2, Kekule believed that each atom has a certain amount of chemical strength. Kekule called this force atomicity, which should be translated as "atomization force" or "atomic force". Later, it was called valence. Carbon is tetravalent, hydrogen and chlorine are monovalent, and oxygen is divalent. If a short line represents one price, CH3Cl can be expressed by the following four equations:
In fact, CH3Cl has only one compound, so they also noticed that the four valence bonds of carbon atoms are equal.

(2) Carbon atoms form bonds through self bonding: In the history of organic chemistry, typology holds an important position. Its founder Gerhardt (C., 1853) believed that organic compounds are derived by replacing one hydrogen with an organic group in four types - hydrogen type, hydrochloric acid type, water type, and ammonia type - for example, they are replaced by an ethyl group:
This theory played a significant role in establishing organic compound systems, summarizing various chaotic compounds into one system and predicting many new compounds according to this theory, which were later discovered one by one. Kekule proposed a new type of methane based on this, which includes other hydrocarbons such as ethane, which is methyl methane。
The two equations above represent the types, numbers, and order of atoms in a molecule, and are called structural equations. Each line in the construction represents a valence key, which is called a key. If two atoms are each bonded by a valence bond, this bond is called a single bond; In some compounds, two or three valence bonds can also self bind to each other, which are called double or triple bonds; Carbon atoms can also combine to form rings:
The left equation shows four carbon atoms connected in a straight line, known as a straight chain. The right equation shows three carbon atoms forming a chain, with the middle carbon atom connected to another carbon atom to form a branched chain, known as a branched chain. These are two isomers, known as carbon frame isomers. C4H10 cannot write the third equation, and experiments have shown that there is no third isomer present. After being tested by thousands of compounds, these two basic rules are used without error in the vast majority of situations. Therefore, the achievements of Kekule and Guber in organic chemistry are indelible.
G é rard and Kekule's view on structure at that time believed that molecules were a "building" composed of various atoms, like wooden frames and bricks. Not only were they connected in a certain order, but the "building" also had a certain style and image. This was a molecular structure of the architectural view. Although this view was correct, such structures were difficult to determine at that time. Therefore, they believed that the structure of this "building" was a tool for reaction and could not be determined by chemical reactions. It was not until a hundred years later that X-ray diffraction technology made significant progress and reached the stage of indirectly photographing molecules, which confirmed this viewpoint.

2. Butlerov's (1861) chemical structure concept
Buterelov first proposed the concept of chemical structure, believing that the chemical properties of a compound depend on the relationships and binding modes between atoms in the molecule that are bound to each other in a certain number. This relationship represents the chemical structure of the compound. The chemical structure not only represents a tool for the reaction of a compound, but also a pattern of the mechanical positions of each atom in the molecule, and indicates certain chemical relationships among the atoms in the molecule, which can be inferred from the chemical properties of the compound. The properties of a molecule depend on the properties, quantity, and relationships of the atoms that make up it. If these are different, it can cause differences in properties. Therefore, the chemical structure of a molecule can be determined from its chemical properties, and conversely, the chemical properties of a molecule can be understood and predicted from its chemical structure. For a long time, people have used chemical properties to determine the chemical structure of molecules. Due to the continuous development of new technologies, the understanding of structures is deepening, and now both chemical structures and molecular architectural images are gradually being mastered by people.