For a good governance of an association, a set of rules and regulations are required. Similarly, for chemistry, certain rules have been stated to make the study easier and less complex.
As it is said that all matters in this universe have a tendency to attain stability; and to do so the process shall start at the molecular level.
So that’s how it is:
Hence the Octet Rule states that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas.
Octet rule gives the number of bonding electrons, valence electrons, oxidation number and the number of bonds that would be formed. Hence and the idea of stability could be generated and by knowledge of valence electrons, the stability could be predicted. Yet the stability could not be confirmed as for transition elements, due to the shielding effect of their “d” orbital electrons a steric hindrance is offered which octet rule could not justify. Hence an idea of stability could be generated, but solely one can’t rely on octet rule for the stability of an atom.
1. The octet rule is not applicable for non-metal after silicon as those elements have a tendency to expand their octet and store more than 8 electrons.
PF5, SF6, H2SO4.
In PF5, the P atom has 10 electrons in the valence shell; similarly
For SF6 the S atom has 12 electrons in its valence shell.
And for H2SO4 the S atom has again 12 atoms in the valence shell.
2. Atoms with an odd number of electrons do not follow the octet rule. For Example NO and NO2. The N atom in both molecules has 7 electrons in their valence shell. Hence the octet is incomplete.
3. For some molecules, central atom can’t have 8 electrons. For example BeCl2 and BCl3. Be has only 4 electrons in valence shell and B has 6 electrons. Hence the octet rule is not valid in such cases.
Hence the OCTET rule has been a great rule for determining the valence electrons and determining the number of bonding electrons in a molecule.
Through octet rule we determined the number of valence electrons and bonding electrons, now to see how bonding happens a sharing of electrons take place we need to draw some basic structures. One of the structures is the Lewis Dot Structure.
The diagrammatical representation of bonds being formed between valence shell-electrons and the lone pair electrons present in an atom is termed as Lewis Dot Structure.
NOTE- one electron is shared with only one electron of the other element and as the number of electrons being shared with each other reaches 8 stop connecting the dots and the Lewis Dot Structure is Ready.
From the above-mentioned Lewis Structure of CCl4, it is Clearly depicted that Carbon being tetravalent has made 4 covalent bonds with Chlorine, where each Chlorine Atom has shared one electron with each Carbon electron.
(http://weknownyourdreamz.com/symbols/chlorine-electron-dot-symbol.html) refer the link for more images.
As in Lewis structure, the number of bonding and non-bonding electrons could clearly be determined. Now the electrons that don’t take part in the bonding would induce charges on the molecule formed, yet the molecule remains stable and electrically neutral. This stability is explained by the concept of Formal Charge.
Hence formal Charge is defined as the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity.
FC = V - N - (B/2)
Hence, with the Lewis Dot Structure, the nature of a bind could easily be predicted and further calculations involving electrons could also be carried out.