Quantum mechanics is the branch of physics that describes the behavior of systems on very small length and energy scales, such as those found in atomic and subatomic interactions. It is essential for understanding certain concepts that classical physics cannot explain, such as the discrete nature of small-scale interactions, wave-particle duality, the uncertainty principle, and quantum entanglement. Quantum mechanics forms the basis for our understanding of many phenomena, including chemical reactions and radioactive decay, as well as all computers and electronic devices today.
While the roots of quantum mechanics can be traced to experiments performed in the 19th century, the theory began to emerge when Max Planck proposed a "quantum hypothesis" to explain the energy spectrum of black body radiation in 1900. In 1905, Albert Einstein suggested that light is composed of discrete packets (quanta) in order to explain the photoelectric effect. A decade later, Niels Bohr proposed a model of the atom in which angular momentum is quantized. Eventually, the mathematical formalism that became known as quantum mechanics was developed in the 1920s and 1930s, with Erwin Schrodinger's discovery of wave mechanics and Werner Heisenberg's discovery of matrix mechanics.
The uncertainty principle
As a result of the wave nature of the electron, the position of the electron can never be precisely known. Whenever it is attempted to be measured, knowledge of the electron's velocity is lost. Hence, there is an inherent uncertainty that prevents precisely measuring both the position and the momentum simultaneously. This is known as the Heisenberg Uncertainty Principle.
An important aspect of Quantum Mechanics is the predictions it makes about the radioactive decay of isotopes. Radioactive decay processes, controlled by the wave equations, are random events. A radioactive atom has a certain probability of decaying per unit time. As a result, the decay results in an exponential decrease in the amount of isotope remaining in a given sample as a function of time. The characteristic time required for 1/2 of the original amount of isotope to decay is known as the "half-life" and can vary from quadrillionths of a second to quintillions of years.
Concepts in quantum mechanics
Important contributors to quantum mechanics
For an excellent discussion of quantum mechanics, see: http://www.chemistry.ohio-state.edu/betha/qm/