Octet Rule and its Limitations

Chemistry Art Integration project on the Octet Rule and its Limitations: Abegg's rule, octet rule, achievements, Langumir cubical atomic models, Limitations
Class XI NCERT Chemistry

1. Submitted by:
Sharan Keshav (Roll no: 30)
Rajiv Sukesh (Roll no: 20)
Anirudh A G (Roll no: 2)
Ilamparithi (Roll no: 7)
(Students of XI A8)
The Octet Rule and its Limitations

2. ABEGG'S RULE
•Before octet rule was theorized, there were always methods to approach electron
configuration. The most famous wasRichard Abegg's attempt in 1904 known as the
Abegg's rule.
•It stated that the difference between the maximum positive and negative valences of
an element is frequently eight.
•Abegg's rule was the cornerstone for various chemical achievements of his time and
also enabled us to proceed forward with the octet rule.
•The rule also had its own exceptions and limitations (such as the odd and even
fallacy and hydrogen's exception).
(Richard Abegg)
This is a Sulphur Atom. It has 6 Valence
electrons. It can gain a maximum of 2
electrons (H2S) and lose a maximum of
6 electrons (H2SO4). Sum of their
absolute values 6 + 2 = 8

3. OCTET RULE
•Abegg'srule as discussed before opened a new wave of research into the valence
electrons of an atom. In 1916, Kösseland Lewis made a breakthrough with help of the
octet rule.
•It stated that, atoms can combine either by transfer of valence electrons from one
atom to another (gaining or losing) or by sharing of valence electrons in order to
have an octet in their valence shells. “Octet” means a group of 8
•It was a remarkable breakthrough in science which helped redefine our ways of
thinking about chemistry as a whole.
Walther Kossel G. N. Lewis

4. ACHIEVEMENTS
While it is clear about how important the octet rule was, here are some of its commonly
known achievements:
•Theoctet rulesuccessfully explains the formation of chemical bonds depending
upon the nature of the element.
•It is used to determine stability in atoms.
•Theoctet rulehelps to figure out how atoms will bond with one another

5. LIMITATIONS(LANGMUIR)
•Irving Langmuirwas a 38-year-old research chemist atGeneral Electric when he
firstbegan to write on the subject ofLewis’ electron pair bonding
•He went on to extend the theory --Langmuir’s postulated shells sequence of 2, 8, 8, 18,
18,and 32 and the introduction of a static molecule
•Unfortunately his ambitions were short-lived as many physicists such as Bohr began to
identify flaws in his papers and this lead to the hole in the octet rule
•It also sparked a huge debate between Lewis and Langmuir about electron
deficienthydrides
Irving Langumir
Langmuir’s cubical atom models
Covalent Bonding (A->B->C)

6. LIMITATIONS
The octet rule had many flaws. The most significant being:
•The Incomplete octet: In certain molecules such asBeH
2,BeCl
2,BH
3,BF
3, the central atom has less
than 8 electrons in its valence shell, yet the molecule is stable.
•The Expanded octet: In certain molecules such asPF
5,SF
6,IF
7,H
2SO
4, the central atom has more than
8 valence electrons, yet the molecule is stable.

7. •The octet rule also failed to predict the shape and the relative stability of the molecules.
•Furthermore therecent quantum mechanical calculations have tended to discountthe importance of
octet expansion and havereinstated the general validity of the octet rule for the p-block elements.
•Also, modern bonding theory has also provided a satisfactory resolutionof the problems associated
with both electron-deficient and electron-rich species and has done soin such a way that neither
Langmuirnor Lewis was correct.
LIMITATIONS

8. In conclusion, when all is said and done, this story and breakthrough was for the
history books (and in this case chemistry books as well). While the theory is not
as useful and relevant as it initially was, it lead to a large scientific diversion
about thinking on the bonding of atoms and valence electrons as a whole.
Thank You!