Why do certain compounds defy the octet rule?

Certain compounds defy the octet rule due to the presence of odd-electron species, expanded octets, or incomplete octets.

The octet rule is a chemical rule of thumb that reflects the theory that atoms of main-group elements tend to combine in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. However, there are several exceptions to this rule, primarily due to the presence of odd-electron species, expanded octets, or incomplete octets.

Odd-electron species, also known as free radicals, are molecules or ions with an unpaired electron. They do not follow the octet rule as they have an odd number of electrons, making it impossible to distribute them evenly among the atoms in a way that each atom achieves a full octet. Examples of such species include nitric oxide (NO) and the methyl radical (CH3).

Expanded octets occur in molecules or ions with more than eight valence electrons around the central atom. This is possible for elements in the third period and beyond, as they have d orbitals available for bonding in addition to the s and p orbitals. This allows them to accommodate more than eight electrons. Examples include phosphorus pentachloride (PCl5) and sulfur hexafluoride (SF6).

Incomplete octets are found in molecules or ions with fewer than eight electrons around an atom. This is often the case with elements that have fewer than four valence electrons, such as hydrogen, helium, lithium, beryllium, and boron. These elements often form stable compounds with incomplete octets. For example, in boron trifluoride (BF3), the boron atom has only six electrons in its valence shell.

In conclusion, while the octet rule is a useful guideline in understanding the structure and behaviour of many compounds, it is not universally applicable. The presence of odd-electron species, expanded octets, or incomplete octets in certain compounds can lead to deviations from the octet rule.