Bohr–Bury rules of electronic configuration: Bohr and Bury simultaneously proposed the following rules for the distribution of electrons in different shells.

• Rule 1: The maximum number of electrons that can be accommodated in a shell is equal to 2n² where ‘n’ is the quantum number of the shell (i.e., the serial number of the shell from the nucleus).
• Rule 2: Electrons may go into a new outer shell although the inner shells are incompletely filled.
• Rule 3: The outermost shell cannot have more than 8 electrons and the next inner, i.e., the penultimate, shell cannot have more than 18 electrons.
• Rule 4: Even if the capacity of the penultimate shell exceeds 8, it cannot have more than 8 electrons unless there are 2 electrons in the outermost shell. And it cannot have more than 9 electrons unless the anti–penultimate shell (i.e., the shell immediately preceding the penultimate shell) is completely filled. In such cases, the outermost shell can have more than 2 electrons only after there are 18 electrons in the penultimate shell.

All the noble gas atoms, except those of helium, have 8 electrons in their outermost shell. G.N Lewis put forth the idea that the existence of eight electrons in the valence shell makes the electronic arrangement especially stable and that is why the noble gases are chemically inactive. The group of eight electrons in the valence shell is called an octet and Lewis formulated the octet rule, according to which an atom, other than hydrogen and lithium, tends to form bonds until it is surrounded by eight valence electrons.

Chemically active elements do not have an octet of electrons in the valence shell of their atoms. Their activity arises from their tendency to achieve the octet by forming bonds either with atoms of their own type or with atoms of other elements.

A hydrogen atom, having only the K shell, can achieve a maximum of two electrons, i.e., a duplet, like helium. A lithium atom can also achieve only a duplet and never an octet.

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