BOHR&BURY RULE

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.
ELECTRONIC CONFIGURATION OF NOBLE GASES
The atoms of noble gases are very similar in their electronic configuration, as we can see from the above table.
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.

ELECTRONIC CONFIGURATION

           


                                   

"d" orbitals

                                                

Thomson's Model

Thomson's Model

According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically. The electrons must be held by the positive charges by electrostatic forces. The mutual repulsions between the electrons are balanced by the force of attraction towards the centre of the sphere. In an atom with a single electron like the hydrogen atom, the electron must be situated at the centre of the positive sphere. In an atom with two electrons, like the helium atom, the two electrons must be symmetrically situated on opposite sides of the centre at a distance equal to half the radius of the positive sphere. In the three-electron system, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which is equal to the radius of the sphere. Proceeding in this manner, Thomson could explain the arrangement of electrons ranging from 1 to 100 inside the positive sphere. This model was also called 'the plum pudding model'. The electrons are like plums in a pudding (positive charge). The seeds in a watermelon fruit can be given as a parallel comparison.

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With this model he tried to explain the observed spectra of elements. According to him, the vibrations of electrons would give rise to electromagnetic energy radiated. He could explain with some success the process like chemical reaction, radioactivity etc.

This model had many drawbacks. The chief among them are:

(i) Since the weight of an electron is about a thousandth part of a hydrogen atom, it would mean that a single atom, especially of the heavier elements, would contain many thousand electrons. But J.J. Thomson himself found that the number of electrons in an atom cannot be greatly different from the atomic weight.

(ii) According to this model, hydrogen can give rise to only one spectral line, contrary to the observed fact of several lines.

(iii) This model could not explain the large angle scattering of alpha particles by thin metal foils.