Why do expanded octets occur in certain elements?

Expanded octets occur in certain elements because they have d-orbitals available for bonding in their valence shell.

In more detail, the octet rule states that atoms tend to combine in such a way that they each have eight electrons in their valence shells, giving them the same electronic configuration as a noble gas. This rule is based on the observation that atoms of main group elements tend to bond in such a way that each atom has eight electrons in its valence shell. However, this rule is not universal and there are exceptions.

One of these exceptions is the expanded octet, which refers to the scenario where an atom has more than eight electrons in its valence shell. This is possible for elements in the third period and beyond, as they have d-orbitals available in their valence shell for bonding. The d-orbitals can accommodate up to ten electrons, allowing these elements to form more than four bonds and hence have more than eight electrons in their valence shell.

For example, phosphorus (P), sulfur (S), and chlorine (Cl) are elements that commonly form expanded octets. Phosphorus can form five bonds, as in the molecule PCl5, and sulfur can form six bonds, as in the molecule SF6. In these cases, the central atom (P or S) has ten and twelve electrons in its valence shell, respectively, exceeding the octet.

The ability to form expanded octets provides these elements with additional possibilities for bonding and molecular structure. It also allows them to form compounds with higher coordination numbers, which refers to the number of atoms or ions immediately surrounding a central atom in a complex or coordination compound.

In summary, expanded octets occur in certain elements because these elements have d-orbitals available in their valence shell for bonding, allowing them to accommodate more than eight electrons and form more than four bonds. This is a notable exception to the octet rule and is a key factor in the bonding and molecular structure of these elements.