The history of chemical substance
The concept of "chemical substances" was established in the late 18th century by the chemist Joseph Proust after studying the composition of some pure compounds, such as basic copper carbonate. He concluded that "all samples of a compound have the same That is to say, calculated by mass, the proportions of elements in the compounds in all samples are the same.” This is the law of constant synthesis. Later, with the development of chemical synthesis methods, especially in the field of organic chemistry; in the field of analytical chemistry The discovery of more chemical elements and new technologies for separating and purifying elements and compounds from chemicals has led to the establishment of modern chemistry. The definition of this concept is the same as found in most chemistry textbooks. However, this definition is somewhat controversial, mainly because a large number of chemical substances in the chemical literature need to be indexed.

The isomers shocked the early researchers because the composition of the isomers is exactly the same, but the configuration (arrangement) of the atoms is different. For example, people had a lot of speculation about the chemical properties of benzene until Friedrich August Kekulé described the correct structure. Similarly, the concept of stereoisomerism—atoms have rigid three-dimensional structures and can therefore form isomers that differ only in their three-dimensional arrangement—is another key step in understanding the concept of different chemical substances. For example, tartaric acid has three different isomers, a pair of diastereomers and one diastereomer form two enantiomers. https://www.echemi.com/

Chemical elements
An element is a chemical substance composed of specific atoms, so it cannot be decomposed or converted into another element through a chemical reaction, although it can be converted into another element through a nuclear reaction. This is because all atoms in an element sample have the same number of protons, although they may be different isotopes and the number of neutrons is also different.

As of 2019, there are 118 known elements, of which about 80 are stable-that is, they will not become other elements due to radioactive decay. Some elements can appear as more than one chemical substance (allotrope). For example, oxygen exists in the form of diatomic oxygen (O2) and ozone (O3). Most elements are classified as metals. These elements such as iron, copper and gold have a unique luster. Metals generally have good electrical and thermal conductivity, and have ductility and ductility. There are about a dozen elements, such as carbon, nitrogen and oxygen, which are classified as non-metallic elements. Non-metals lack the metal properties described above, and they also have high electronegativity and a tendency to form negative ions. Certain elements, such as silicon, are sometimes similar to metals and sometimes similar to non-metals, and are called metalloids.