How to Know if a Lewis Acid Has an Empty Orbital

Study how electrons arrange themselves around the nucleus of the atom. Electrons are arranged in shells and each shell must be completed before the next shell is started. The first three shells are called p, d and f, and each of these are composed of orbitals, each of which holds two electrons. The p shell has one orbital -- two electrons, the p shell has four orbitals --- eight electrons and the d shell has nine orbitals -- 18 electrons. The charge of the nucleus determines how many electrons the atom will have. For example, oxygen has eight electrons. The p shell is full and the s shell is almost full -- it has one empty orbital.

Observe how Lewis acids and Lewis bases fit together to form Lewis complexes. Usually, acids and bases snap together because acids are one electron short of having a complete shell and bases have one extra electron after completing a shell. The complex shares this extra electron and it seems like all the shells are complete. Lewis acids and bases are just special cases where pairs of electrons are shared. This sharing of pairs of electrons may be because the Lewis acid has an empty orbital or it may be because the Lewis acid has a couple of double bonds -- atoms sharing two electrons -- that can be converted to single bonds if the Lewis acid could combine with a Lewis base.

Recognize the nomenclature difference in the two types of Lewis acids. If the Lewis acid has an empty orbital, the formula will be marked with a "+" sign showing that it has an empty orbital that needs to be filled to make up a shell. If the Lewis acid has double bonds that could be converted to single bonds if there were an additional pair of electrons, there will be no sign. Lewis bases always have a "-" sign to denote there is a surplus of electrons after the last completed shell.

Detect a Lewis acid with an empty orbital by finding any structure that has exactly one empty orbital short of a complete shell.