Quantum mechanics is the branch of physics that deals with the behavior of matter and energy at the smallest scales, such as atoms and subatomic particles. It was developed in the early 20th century to explain phenomena that classical mechanics couldn't, like the behavior of electrons in atoms and the nature of light. However, classical mechanics failed to explain the motion of subatomic particles like electrons, protons, neutrons etc.
Max Plank proposed the Quantum theory to explain Blackbody radiation. Einstein applied it to explain the Photo Electric Effect. In the mean-time, Einstein’s mass – energy relationship (E = mc2) had been verified in which the radiation and mass were mutually convertible. Louis de-Broglie extended the idea of dual nature of radiation to matter, when he proposed that matter possesses wave as well as particle characteristics
The classical mechanics and the quantum mechanics have fundamentally different approaches to solve problems. In the case of classical mechanics, it is unconditionally accepted that position, mass, velocity, acceleration of a particle can be measured accurately, which of course true in day to day observations. In contrast, the structure of quantum mechanics is uilt upon the foundation of principles which are purely probabilistic in nature. As per the fundamental assumption of quantum mechanics, it is impossible to measure simultaneously the position and momentum of a particle, whereas in the case of classical mechanics, there is nothing which contradicts the measurements of both of them accurately.