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Chemistry is defined as the science of matter, different from physics, in terms of matter's chemical structure, reactivity, properties and reactions or transformations. It is the science of matter, its structure, chemical reactivity and properties, and the transformations it undergoes. It is often called "the central science" because it is concerned with the fundamental energy or forces that hold matter together studied in physics and the molecular systems and characteristics of the complex organisms studied in biology (see physics, biology).

Theories of chemical bonds, resulting from the scientific philosophy of chemistry, explains not just the chemical properties of different forms of matter, but also most of the physical ones.

Because of the diversity of matter (which has the atom as the fundamental particle, except for individual protons, neutrons and electrons), chemists are often engaged in the pursuit of studying how atoms interact to form molecules, and how molecules interact with each other. Consequently, chemistry includes the study of microscopic phenomena, such as clusters of atoms and their characteristics on the nanometre scale, and macroscopic phenomena, such as the interaction of proteins and DNA in complex solutions and the properties of new materials.