Quantum Mechanics is the theory of the behavior of all the elementary forms of matter and radiation and their interactions. It's what the entire modern theory of atoms are based. It includes their nucleus, molecules, properties of matter, and the particles of that they are all composed of.

Quantum Mechanics was invented by men trying to understand the structure of atoms. Ernest Rutherford and Niels Bohr invented the idea of the nuclear atom in 1911.The Bohr-Rutherford theory contained several ideas that are useful because all reappear, in one form or another, in modern theory.

The modern Quantum Mechanical model of the atom says an atom has a nucleus surrounded by electrons found in regions called electron clouds. The electron clouds represents the spaces where the electrons are most likely to be found.

The location of an electron in this could depends on how much energy the electron has. Electrons are arranged in energy levels in an electron cloud. The electrons with the lowest energy are closest to the nucleus, and the electron with the highest energy are farther from the nucleus. Because electrons are moving so fast around the nucleus, their exact location can't be determined. Electron clouds represent the entire region of space the electrons could be occupying at any time.

The Quantum Mechanical Model of the atom gives each electron a set of four quantum numbers that describe its probable location. The orbitals determine where the electrons are located. Each orbital is determined by a quantum number. This quantum number can be: l=0 (s-orbital), l=1 (p-orbital), l=2 (d-orbital), l=3 (f-orbital) etc.