Chemistry is the science that studies the composition, structure (types, bonds and spatial arrangement of atoms) and properties of matter and the transformations it undergoes through chemical reactions.

Chemistry is an empirical science, as it studies the matter through the scientific method, that is, by the observation and quantification and, especially, by experimentation. In its broadest sense, chemistry studies the diverse substances that exist on our planet (observable universe) and the reactions that transform them into other substances. Moreover, chemistry studies the chemical structure of the substances at their molecular level. And last but not least, their properties.

Natural processes studied by chemistry involve fundamental particles (electrons, protons and neutrons), composite particles (atomic nuclei, atoms and molecules) or microscopic structures such as crystals and surfaces. If there is an important and representative particle chemistry, this is the electron. One of the greatest achievements of chemistry is to have come to understand that chemical reactions are related to the internal structure and arrangement of electrons in atoms, molecules or solids.

Traditional chemistry began with the study of elementary particles: atoms, molecules, substances, metals, crystals and other aggregates of matter. The interactions, reactions and transformations that are studied in chemistry are usually the result of interactions between atoms, leading to rearrangements of the chemical bonds that hold atoms to each other.

A chemical reaction is the conversion of substances in one or more different substances. The basis for such chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. Can be represented symbolically as a chemical equation, which usually involves atoms as the core particle. The number of atoms on the left and the right in the equation for a chemical transformation must be equal (if it is not equal, the transformation, by definition, is not chemical, it is a nuclear reaction or a radioactive decay). The type of chemical reactions a substance may undergo and the energy changes that may accompany it, are determined by certain basic rules, known as chemical laws.

From the microscopic point of view, the particles involved in a chemical reaction can be considered a closed system that exchanges energy with its surroundings. In exothermic processes, the system releases energy to its surroundings, while an endothermic process can only occur when the environment provides energy to the system that reacts. In most of chemical reactions there is a flow of energy between the system and its influence field, so it may extend the definition of chemical reaction and involve the kinetic energy (heat) as a reactant or product.

Although there are a variety of branches of chemistry, the main divisions are:

Inorganic chemistry. Studies the formation, composition, structure and chemical reactions of the elements and inorganic compounds (formed by atoms other than carbon, with some exceptions).

Organic chemistry or carbon chemistry is the branch of chemistry that studies the molecules that are based on carbon chains containing carbon atoms and other atoms (heteroatoms), also known as organic compounds.

Biochemistry. Studies the chemical reactions in living organisms.

Physical Chemistry. Studies the basics and physical basis of chemical systems and processes. In particular, energy and dynamic aspects of such systems and processes.

Analytical chemistry. Studies the methods of detection (identification) and quantification (determination) of a substance in a sample. It is subdivided into quantitative and qualitative.