What is an example of an incomplete octet?

An example of an incomplete octet is boron trifluoride (BF₃), where boron only has six valence electrons around it.

Understanding Incomplete Octets

The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve a full outer electron shell with eight electrons. However, some elements, notably those found towards the left of the periodic table, can be stable with fewer than eight electrons in their valence shell. This situation is known as an incomplete octet.

Common Examples of Incomplete Octets

Several elements are frequently associated with incomplete octets:

• Hydrogen (H): Stable with only 2 valence electrons (duet rule).
• Beryllium (Be): Stable with only 4 valence electrons. For example, in beryllium chloride (BeCl₂).
• Boron (B): Stable with only 6 valence electrons. Boron trifluoride (BF₃) and boron trichloride (BCl₃) are common examples.
• Aluminum (Al): While aluminum can achieve an octet, it often forms compounds where it has only 6 valence electrons, similar to boron. For example, aluminum chloride (AlCl₃) can exist as a dimer (Al₂Cl₆) where each Al has an octet, but monomeric AlCl₃ is also known and features an incomplete octet.

Boron Trifluoride (BF₃) in Detail

In BF₃, boron forms three single bonds with fluorine atoms. Fluorine, being highly electronegative, readily forms single bonds to achieve an octet. However, boron has only three valence electrons to start with. After forming the three bonds, boron has only six electrons around it. It could theoretically accept a lone pair from one of the fluorines to complete its octet, but this is not energetically favorable in BF₃, making it an example of an incomplete octet. This electron deficiency makes BF₃ a strong Lewis acid (electron acceptor).

Importance of Incomplete Octets

Understanding incomplete octets is important because they influence a molecule's reactivity and properties. Molecules with incomplete octets are often Lewis acids, readily accepting electron pairs from other molecules to achieve a more stable electron configuration.