1869 - Mendeleev: published the periodic table

1. 1800 1803 - J. Dalton: defined atom as the smallest part of a subs. that can participate in a chem. rxn. 1835 - C. Wheatstone: different metals ,different bright lines in their sparks’ emission spectra 1850 1853 - Angstrom 1860 – Kirchoff: showed that there are three types of spectra emitted by objects 1869 - Mendeleev: published the periodic table 1900 1900 – Planck: thermal radiation is emitted from a heated surface in discrete packets of energy called QUANTA. Quantum theory is born. 1904 - J.J. Thomson: Plum–Pudding Model: the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification... 1905 – Einsteininterpreted the photoelectric results by suggesting that the energy in a ight wave is also contained in discrete packets. The particle-like packet of energy is called a PHOTON. 1911 – E. Rutherford proposes the nuclear  atomic model. 1913 - N. Bohr proposes his planetary model of the atom, along with the concept of stationary energy states and accounts for hydrogen spectrum. 1924 - De Broglie generalizes wave-particle duality by suggesting that particles of matter are also wavelike. 1924 -Bose & Einstein find a new way to count quantum particles (Bose-Einstein statistics), and they predict that extremely cold atoms should condense into a single quantum state, later known as a Bose-Einstein condensate. 1925 – Pauli enunciates the exclusion principle. 1926 - Schrödinger develops a second description of quantum physics, called wave mechanics to describe the motion of electrons in a crystal. Under construction 1926 – Fermi and Dirac find that quantum mechanics requires a second way to count particles, Fermi-Dirac statistics, opening the way to SOLID STATE PHYSICS. 1927 - Heisenberg states his UNCERTAINTY PRINCIPLE, that it is impossible to exactly measure the position and momentum of a particle at the same time. 1950