Quantum mechanics is a branch of physics developed in the first part of the 20th century that was highly successful in explaining the behavior of atoms, molecules and nuclei. Developed between 1900 and 1930 and later combined with the general and special theory of relativity, it revolutionized the field of physics. The new concepts, which were the particle properties of radiation, the wave properties of matter, quantization of physical properties and the idea that one can no longer know exactly where a single particle such as an electron is at any one time were necessary to explain all of the new experimental evidence that was available at the time.
At the turn of the 20th century, classical theories were well-established for thermodynamics, optics, electromagnetism, and motion of objects. Classical predictions were very accurate for nearly every system that had been studied, and there was a common belief that all of the conceptual problems in chemistry and physics were essentially solved (although solving the equations for a large complex system could be computationally challenging).
A series of experimental results that could not be explained using classical mechanics ultimately led to the development of quantum mechanics. As a result, much of modern chemistry has been explained using the principles of quantum mechanics: for example, atomic structure, molecular bonding, and all types of spectroscopy are directly related to quantum chemistry. Quantum mechanics has also been applied to develop devices we use every day including semiconductors in the circuitry of our computers and lasers in our CD players.