Hypervalence

Hypervalence refers to the ability of certain atoms, particularly those in the third row and beyond of the periodic table, to form more bonds than their valence electron count would conventionally suggest. This phenomenon challenges the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable configuration of eight valence electrons. Elements like sulfur, phosphorus, and chlorine, which are in periods three or lower, can often exhibit hypervalence. For example, sulfur in sulfuric acid (H2SO4) forms six bonds with oxygen atoms, exceeding the typical maximum of four bonds implied by its six valence electrons.

The explanation for hypervalence is often debated. One common theory involves the participation of d-orbitals in bonding, which are available for elements in the third row and below. These empty d-orbitals can accept electron density from neighboring atoms, allowing the central atom to accommodate more than eight valence electrons. Another perspective suggests that hypervalent compounds can be adequately described using two-electron resonance structures without invoking d-orbital participation. Regardless of the exact theoretical framework, hypervalence is a recognized and important concept in understanding the chemistry of many main group elements. Examples of hypervalent compounds include SF6, PCl5, and IO4-.