General Chemistry • Chemical Bonds
Some covalent molecules are electron-deficient, which means the central atom finishes with fewer than 8 electrons around it. Explore BeCl2, BBr3, and AlI3 to see how bonding, electron-domain geometry, and Lewis-acid behavior are connected.
An incomplete octet means the central atom has fewer than 8 electrons around it in the Lewis structure. This is an accepted exception for certain electron-deficient elements in simple molecules.
Beryllium and boron are common examples because they often form 2 or 3 bonds without enough electrons to reach an octet. Aluminum can also appear electron-deficient in simplified Lewis descriptions.
Add the valence electrons from each atom in the formula (and adjust for charge if an ion is selected). The total is then distributed into bonds and lone pairs, even if the central atom ends up with fewer than 8 electrons.
A central atom with fewer than 8 electrons is electron-poor and can accept an electron pair from another species. That electron-pair acceptance is the definition of a Lewis acid.
Not necessarily; many incomplete-octet molecules are stable under normal conditions. The Lewis structure is a simplified electron-bookkeeping model and the incomplete octet is a recognized rule exception in general chemistry.