What are the exceptions to the octet rule with examples
In 1916, Kössel and Lewis developed an important theory of chemical combination between atoms known as the electronic theory of chemical bonding. Accordingly, atoms can combine either by transferring valence electrons from one atom to another (gaining or losing) or by sharing valence electrons so as to have one octet in their valence shells. This law is known as the Octet rule.
Significance of the Octet Rule
The Octet rule which can explain chemical bonding in different compounds.
Octet rules illustrate the different types of bonds, such as a covalent, electrovalent and coordinated bond.
Limitations Of Octet Rule
An atom's stability is determined by the octet rule. According to this rule, an atom is stable if its outermost shell has a total of 8 electrons. In order for an atom to be stable, the outermost shell must be completely filled. Essentially, this is a special case of the atom stability rule. There is another case of the stability rule of atoms known as the duplet rule. This rule, although not universal, provides useful insight into the structure of most organic compounds. In the modern periodic table, the octet rule mostly applies to the elements of the second and third periods.
There are Mainly three limitations of the Octet Rule:
The Incomplete Octet of the Central Atom
In the case of molecules that have nearly equal ionic and covalent characteristics, the octet rule fails because no bond is completely ionic or completely covalent.
In some molecules, such as LiCl, BeH₂ and BCl₃, there are fewer than eight electrons surrounding the central atom. There is only 1 valence electron in Li, Be, and B. The situation is particularly challenging when it comes to elements with valence shells with fewer than four electrons. BF₃ and AlCl₃ are examples of similar compounds.
Odd-electron Molecules
As for molecules that do not obey the octet rule, such as nitric oxide, NO, or nitrogen dioxide, NO₂, they have odd numbers of electrons.
The Expanded Atom
Bonding is possible on 3d orbitals, which are not available on 3s and 3p orbitals. In fact, an atom is stable once the shell surrounding its outermost part is completely filled. The entire outermost shell in this case (4th period) can hold up to 18 e-. These elements have compounds with valence electrons around their central atoms greater than eight. These compounds are known as expanded octets. PF₅, SF₆, H₂SO₄, and many other coordination compounds are examples of such compounds.
Other Drawbacks of the Octet Theory
The rule of the octet is evidently based on the chemical inertness of the noble gases.
However, some noble gases (e.g. xenon and krypton) also combine with oxygen and fluorine to form compounds such as XeF₂, KrF₂, XeOF₂, etc.
The shape of the molecules does not account for this theory.
It does not clarify how relatively stable the molecules are completely silent about a molecule's energy.
FAQs on Limitations of the Octet Rule in Chemical Bonding
1. What are the limitations of the octet rule?
The octet rule is limited because many atoms do not strictly achieve eight valence electrons in their compounds. The main limitations include:
- Incomplete octet: Some atoms like B in BF3 have fewer than eight electrons.
- Expanded octet: Elements in Period 3 and beyond, such as P in PCl5 or S in SF6, can have more than eight electrons.
- Odd-electron molecules: Molecules like NO and NO2 contain an odd number of valence electrons.
- Transition elements: Their bonding cannot be explained by the simple octet concept.
2. Why does the octet rule fail for some elements?
The octet rule fails because it applies mainly to second-period elements and does not account for incomplete, expanded, or odd-electron configurations. The reasons include:
- Some atoms are stable with fewer than eight electrons (e.g., B in BF3).
- Heavier elements have accessible d-orbitals allowing expanded valence shells (e.g., PCl5).
- Free radicals have an odd number of electrons (e.g., NO).
- The rule does not consider molecular orbital theory or delocalization effects.
3. What is an incomplete octet with an example?
An incomplete octet occurs when an atom is stable with fewer than eight electrons in its valence shell. For example:
- In BF3, boron forms three B–F bonds and has only 6 valence electrons.
- In BeCl2, beryllium forms two bonds and has only 4 valence electrons.
These molecules are stable despite not satisfying the octet rule.
4. What is an expanded octet with an example?
An expanded octet occurs when an atom has more than eight electrons in its valence shell. Examples include:
- PCl5, where phosphorus has 10 valence electrons.
- SF6, where sulfur has 12 valence electrons.
This is possible for elements in the third period or beyond due to available higher-energy orbitals.
5. Why does boron not follow the octet rule?
Boron does not follow the octet rule because it is stable with only six valence electrons in many of its compounds. For example:
- In BF3, boron forms three single covalent bonds.
- This gives boron only 6 electrons around it.
- The molecule remains stable due to electron deficiency and possible coordinate bonding in reactions.
6. Do free radicals obey the octet rule?
Free radicals do not obey the octet rule because they contain an unpaired electron and an odd number of total valence electrons. Examples include:
- NO (nitric oxide)
- NO2 (nitrogen dioxide)
These molecules cannot distribute electrons to give every atom eight electrons.
7. Why does phosphorus form PCl5 if it exceeds the octet?
Phosphorus forms PCl5 because it can expand its valence shell beyond eight electrons. In PCl5:
- Phosphorus forms five P–Cl bonds.
- It has 10 electrons in its valence shell.
- Elements in Period 3 can accommodate expanded valence shells.
This explains why the octet rule is not universally applicable.
8. Does the octet rule apply to transition elements?
The octet rule does not generally apply to transition elements because their bonding involves d-orbitals and variable oxidation states. For example:
- In many transition metal complexes, the metal ion does not have exactly eight valence electrons.
- Bonding is better explained by valence bond theory or crystal field theory.
Thus, the simple octet concept is insufficient for transition metals.
9. What are odd-electron molecules in relation to the octet rule?
Odd-electron molecules are molecules that contain an odd total number of valence electrons and therefore cannot satisfy the octet rule for all atoms. Examples include:
- NO (11 valence electrons)
- ClO2 (19 valence electrons)
At least one atom in these molecules will have an incomplete octet.
10. Is the octet rule universally applicable in chemistry?
The octet rule is not universally applicable because many stable molecules violate it through incomplete, expanded, or odd-electron configurations. It works best for:
- Second-period elements like C, N, O, and F
- Simple covalent molecules such as CH4, NH3, and H2O
However, advanced bonding theories provide a more accurate explanation for many compounds.
