Question

Describe the factors that contribute to the stability of structures in which the central atoms have more than eight valence electrons.

Short answer

Answer: The stability of structures with central atoms having more than eight valence electrons can be attributed to the nature of the central atom (electron configuration and access to d-orbitals), presence of lone pairs, bond strengths (expanded octets), and electronegativity differences between the central and surrounding atoms.

Step-by-step solution

1. Nature of the central atom and its electron configuration

First, it is useful to recognize that not all elements can have more than eight valence electrons. This phenomenon is primarily observed in elements from the third period onwards in the periodic table, which have access to d-orbitals. These d-orbitals provide additional space for electrons, allowing the central atom to accommodate more than eight valence electrons for bonding. For example, sulfur (S) in the sulfate ion (SO4^2-) has 12 valence electrons. Sulfur is from the third period, meaning the 3d-orbitals are available for bonding.

2. Presence of lone pairs and their influence on stability

Lone pairs are electron pairs that belong exclusively to one atom rather than being shared in a bond. In molecules with the central atom having more than eight valence electrons, lone pairs are present on the central atom, which can contribute to the stability of the structure. Lone pairs have the tendency to create electron-electron repulsion with bonding electrons; however, an increase in the effective nuclear charge increases electron affinity, thereby stabilizing the structure by attracting electrons closer to the nucleus.

3. Bond Strengths and Expanded Octets

When there are more than eight valence electrons in the central atom, it forms an "expanded octet." This expanded octet allows the central atom to form multiple bonds with other atoms, increasing bond strength and, therefore, the stability of the structure. For example, in the case of the phosphate ion (PO4^3-), phosphorus (P) can extend its valence shell to 10 electrons, forming strong double bonds with oxygen.

4. Pauling's Electronegativity Principle

Another crucial factor contributing to the stability of such structures is the electronegativity of the central and surrounding atoms. According to Pauling's Electronegativity Principle, the higher the difference in electronegativity between the central atom and its neighboring atoms, the more stable the structure. This increased electronegativity difference results in stronger ionic character, leading to greater stability. For example, xenon hexafluoride (XeF6) is an example of a stable structure where the central atom has more than eight valence electrons. Fluorine (F) is the most electronegative element, and xenon (Xe) is a noble gas, having a significant difference in electronegativity values. Considering the factors discussed above, the stability of structures with central atoms having more than eight valence electrons can be attributed to the nature of the central atom, presence of lone pairs, bond strengths, and electronegativity differences.