Bohr and Bury together suggested certain rules to show how electrons are distributed in different orbits. These rules have to be followed for writing the number of electrons in different energy levels. First rule: The maximum number of electrons present in a shell is given by the formula 2n², where ‘n’ is the orbit number or energy level index (1, 2, 3…). Hence, the maximum number of electrons that different shells can accommodate is as follows:  First orbit or Kshell can accommodate maximally 2 x 1² = 2 electrons.
 Second orbit or Lshell can accommodate maximally 2 x 2² = 8 electrons.
 Third orbit or Mshell can accommodate maximally 2 x 3² = 18 electrons.
 Fourth orbit or Nshell can accommodate maximally 2 x 4² = 32 electrons.
 Second rule: The maximum number of electrons that can be accommodated in the outermost orbit is eight.
 Third rule: Electrons cannot be filled in the outer shell until the inner shells are completely filled. This means that shells are filled in a stepwise manner, starting from the inner shell.

According to the Bohr’s atomic model, the electrons revolve around the nucleus in certain orbits. These electrons are distributed in various orbits according to a scheme proposed by Bohr and Bury. The Bohr and Bury scheme gives the following rules for the distribution of electrons in various orbits.
(i) The maximum number of electrons which can be accommodated in a given orbit is given by the formula 2n^{2}, where not is the quantum number of the orbit. Thus and so on.
(ii) The outermost orbit can have at the most 8 electrons, and the next inner to the outermost (penultimate) can have at the most 18 electrons, (if permissible in accordance with rule i).
(iii) It is not always necessary to fill up an orbit completely before starting the next higher orbit.
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