2.21 Atomic Physics
Atomic physics began as a philosophical concept in ancient philosophy, but the discovery of spectral lines at the beginning of the 19th century led eventually to the Bohr atom model, which provided the theoretical basis for chemistry and spectroscopy. At the same time, many chemical reactions showed that matter did indeed behave as if it were made up of atoms, and the number of irreducible chemical elements started to grow until Dmitri Mendeleev (1834-1907 AD) published the first widely recognized periodic table in 1869. By the beginning of the 20th century, some physicists performed various experiments on electromagnetism and radioactivity, that showed the existence of what later became recognized as subatomic (or elementary) particles, such as electrons, protons and neutrons.
Since the Second World War, both theoretical and experimental fields have advanced at a rapid pace, especially after the progress in computing technology, and the advanced accelerators and detectors. Atomic physics primarily considers atoms in isolation, but it also widely interfere with nuclear physics.