Quantum mechanics was initially developed to provide a better explanation and description of
the atom, especially the differences in the spectra of light emitted by different isotopes of the
same chemical element, as well as subatomic particles. In short, the quantum-mechanical atomic
model has succeeded spectacularly in the realm where classical mechanics and electromagnetism
falter.
Broadly speaking, quantum mechanics incorporates four classes of phenomena for which
classical physics cannot account:
quantization of certain physical properties
quantum entanglement
principle of uncertainty
waveparticle duality