Why are some compounds considered hypervalent?

Some compounds are considered hypervalent because they contain atoms that form more bonds than would be expected by the octet rule.

The term 'hypervalent' is used to describe molecules and ions that contain main group elements forming more than the usual number of bonds. This is typically more than would be expected by the octet rule, which states that atoms are most stable when their outermost shell is full, usually with eight electrons. Hypervalency is a common feature of many compounds of silicon, phosphorus, sulfur, and chlorine, among others.

The concept of hypervalency was first introduced in the early 20th century as a way to explain the structures of certain compounds that seemed to violate the octet rule. For example, phosphorus pentachloride (PCl5) is a hypervalent molecule because the phosphorus atom forms five bonds, resulting in ten valence electrons. According to the octet rule, phosphorus should form only three bonds, as it has five valence electrons and needs three more to complete its octet.

Hypervalency is possible due to the availability of empty d orbitals in the third and higher periods of the periodic table. These d orbitals can participate in bonding, allowing atoms to expand their valence shell and form more than four bonds. This is known as the 'expanded octet' or 'hypervalent' rule.

However, the concept of hypervalency has been a subject of debate among chemists. Some argue that the idea of d orbital participation is not necessary to explain the structures of hypervalent compounds. They suggest that these structures can be explained using a combination of resonance and ionic structures, without invoking d orbitals. Despite this controversy, the term 'hypervalent' is still widely used in the field of chemistry to describe compounds that form more bonds than would be expected by the octet rule.