Short Answer

Hypervalency is a condition in which an atom forms more than eight electrons around itself, meaning it expands its octet. This usually happens in elements from the third period and beyond because they have empty d-orbitals that can participate in bonding. As a result, these atoms can form five, six, or even seven bonds.

Examples of hypervalent molecules include phosphorus pentachloride (PCl₅), sulfur hexafluoride (SF₆), and chlorine trifluoride (ClF₃). Hypervalency helps explain why some molecules do not follow the octet rule and still remain stable.

Detailed Explanation :

Hypervalency

Hypervalency refers to the ability of certain atoms to form compounds in which they have more than eight electrons in their valence shell. In simple terms, while the octet rule says that atoms prefer to have eight electrons around them, hypervalent atoms can hold 10, 12, or even 14 electrons. This expanded valence capacity is possible mainly in elements from the third period and below because they have empty d-orbitals available for bonding.

The concept of hypervalency is important because it explains why many stable molecules do not obey the octet rule. Several everyday chemical substances, especially those involving phosphorus, sulfur, and halogens, exhibit hypervalency.

1. Why Hypervalency Occurs

Hypervalency occurs because:

• Some atoms have vacant d-orbitals in their valence shell that can be used to form additional bonds.
• Larger atoms have more space around them to accommodate extra electron density.
• The central atom may form more than four bonds to achieve stability.

In the third period (n = 3) and beyond, elements like P, S, Cl, Br, and I can use their 3d, 4d, or 5d orbitals to expand their octet, allowing them to form five or six bonds.

2. Examples of Hypervalent Molecules

Several common compounds show hypervalency:

1. Phosphorus pentachloride (PCl₅)

• Phosphorus forms five bonds
• Total electrons around P = 10
• Molecular geometry: Trigonal bipyramidal

2. Sulfur hexafluoride (SF₆)

• Sulfur forms six bonds
• Total electrons around S = 12
• Molecular geometry: Octahedral

3. Chlorine trifluoride (ClF₃)

• Chlorine forms three bonds and has two lone pairs
• Total electrons around Cl = 10
• Molecular geometry: T-shaped

These examples show how atoms can exceed the octet yet remain stable and predictable in shape.

3. Relationship Between Hypervalency and the Octet Rule

The octet rule works well for second-period elements (C, N, O, F) because they do not have d-orbitals and cannot expand their octet. However, third-period elements and beyond have access to d-orbitals, which allow them to accommodate more electrons.

Thus:

• Second-period elements = cannot be hypervalent
• Third-period elements and higher = can be hypervalent

Hypervalency helps explain exceptions to the octet rule, including:

• P in PCl₅
• S in SF₆
• Xe in xenon fluorides (XeF₂, XeF₄, XeF₆)

4. How Hypervalency Connects to Molecular Shapes

Hypervalent molecules often correspond to higher-coordination geometries predicted by VSEPR theory:

• AX₅ → trigonal bipyramidal (PCl₅)
• AX₆ → octahedral (SF₆)
• AX₃E₂ → T-shaped (ClF₃)
• AX₄E₂ → square planar (XeF₄)

The ability to form five or six bonds allows molecules to adopt shapes that are impossible for strictly octet-abiding elements.

5. Modern View of Hypervalency

Modern chemistry offers two main explanations:

1. d-Orbital Participation (Classical View)

• Empty d-orbitals help expand the valence shell.

2. Delocalized Bonding (Modern Viewpoint)

• Bonding is spread over the entire molecule (three-center, four-electron bonds).
• Does not require heavy use of d-orbitals.

Both models help explain molecular structures like PCl₅ and SF₆.

6. Importance of Hypervalency in Chemistry

Hypervalency plays a key role in:

• Describing advanced molecular structures
• Understanding the bonding in many inorganic molecules
• Explaining the shapes and stability of large molecules
• Predicting reactivity and polarity
• Describing expanded octet elements in coordination chemistry

Without hypervalency, many chemical compounds would be difficult to explain using only simple bonding models.

Conclusion

Hypervalency is the ability of certain atoms to hold more than eight electrons in their valence shell. It occurs in elements from the third period and beyond due to the presence of empty d-orbitals and larger atomic sizes. Hypervalent molecules such as PCl₅, SF₆, and ClF₃ remain stable despite violating the octet rule. Hypervalency helps explain many molecular shapes, bonding patterns, and chemical behaviors that cannot be understood using basic octet concepts alone.