Incomplete Octet

There are certain atoms of certain elements that can exist in stable compounds forming bonds with less than eight valence electrons.  When this occurs, the atom of the element within the molecule is said to contain an incomplete octet.  The common examples of such elements are hydrogen (stable with only 2 valence electrons), beryllium (stable with only 4 valence electrons) and boron and aluminum (stable with only 6 valence electrons).   For hydrogen 2 valence electrons give it a noble gas structure (like He) so this is much like the octet rule for everything else below period 1.   But covalent compounds in groups 2 and 3 can form stable compounds in which the valence electrons are not in the noble gas structure.   However, for these compounds you will find that they do form compounds that minimize formal charge.

The exceptions are fairly straight forward to remember. Hydrogen makes one bond (group 1). Beryllium makes two bonds (it is the only element in group 2 to make covalent compounds). The group 3 elements boron and aluminum make three bonds. How many bonds do carbon (and the other group four elements typically make)? Four. Why do these trends generally hold? The elements in the same group have similar electronic structure and thus have similar chemistry.

The following video highlights these three exceptions to the octet rule.