Final answer:
Electron-deficient compounds have a central atom with fewer electrons than required for an octet, violating the octet rule. They are commonly found in compounds of beryllium and boron, and because they can accept electrons, they function as Lewis acids.
Step-by-step explanation:
An electron-deficient molecule is a compound that has fewer electrons than necessary for achieving a complete octet in the valence shell of its central atom. These molecules represent a violation of the octet rule. A classic example is compounds of beryllium and boron. For instance, beryllium typically forms two covalent bonds resulting in only four valence electrons; meanwhile, boron can form three covalent bonds with six valence electrons.
Molecules that are electron-deficient are considered Lewis acids because they can accept electrons to fulfill their octet. This is in contrast to hypervalent molecules, which have more than an octet of electrons around the central atom.