Chronology Of Atomic Structure

CHRONOLOGY OF ATOMIC STRUCTURE

1.       Dalton(1808)        :         Discovery of atom

2.       Julius Plucker (1859)     :         First discoverer of cathode rays

3.       Goldstein(1886)   :         Discovered anode rays and proton

4.       Sir.J.J.Thomson(1897)  :         Discovered electron and determined

                             charge/mass(e/m) ratio for electron

5.       Rutherford(1891) :         Discovered nucleus and proposed

                             atomic model

6.       MaxPlanck(1901) :         Proposed quantum theory of radiation

7.       RobertMillikan(1909)    :         Determined charge of an electron

8.       H.G.J.Mosely(1913)      :         Discovered atomic number

9.       Niels Bohr(1913) :         Proposed a new model of atom

10.     Clark Maxwell(1921)     :         Electromagnetic wave theory

11.     de-Broglie(1923)  :         Established wave nature of particles

12.     Pauli(1927) :         Discovery of neutrino

13.     Werner Heisenberg(1927) :      Uncertainty Principle

14.     James Chadwick(1932)  :         Discovery of neutron

15.     Anderson(1932)   :         Discovery of positron

16.     Fermi(1934)         :         Discovered antineutrino

17.     Hideki Yukawa(1935)   :         Discovered mesons

18.     Segre(1955)          :         Discovered antiproton

19.     Cork and Association(1956) : Discovered antineutron

Progress of Atomic Models

          In 1803, John Dalton, proposed his atomic theory. He suggested that atoms were indivisible solid spheres.

          J.J.Thomson proposed that an atom was a solid sphere of positively charged material and negatively charged particles, electrons were embedded in it like the seeds in a guava fruit. But later this concept was proved wrong.

          Rutherford suggested the planetary model, but this model was rejected.

          In 1913, Neils Bohr proposed that electrons revolve around the nucleus in a definite orbit with a particular energy. Based on the facts obtained from spectra of hydrogen atom, he introduced the concept of energy levels of atom.

          In 1916 Sommerfeld modified Bohr's model by introducing elliptical orbits for electron path. He defined sub energy levels for every major energy level predicted by Bohr.

          The concept of Quantum numbers was introduced to distinguish the orbital on the basis of their size, shape and orientation in space by using principal, azimuthal, magnetic and spin quantum numbers.

          From the study of quantum numbers, various rules are put forward for filling of electrons in various orbitals by following

          Aufbau principle

          Pauli exclusion principle and

          Hunds rule of maximum multiplicity.

          In 1921 Burry and Bohr gave a scheme for the arrangement of electrons in an atom. Further the nature of electron (s) is studied.