Video Transcript
The following equation describes
the formation of the ammonia boron trifluoride compound. The Lewis acid term can be used to
fill at least one of the three empty boxes. Which of the empty boxes should be
filled with the Lewis acid term? (A) Box 1, (B) box 2, (C) box 3,
(D) boxes 1 and 3, (E) boxes 2 and 3.
In 1923, the chemist Gilbert Lewis
developed a new description for acids and bases. He defined a Lewis acid as a
species capable of accepting a pair of donated electrons, forming a bond. This is possible because a Lewis
acid is electron deficient and has vacant orbitals in its valence shell that can
accommodate more electrons.
Now, let’s clear some space to
define a Lewis base. A Lewis base is defined as a
species capable of donating a pair of electrons, forming a bond. Lewis bases can donate pairs of
electrons because they are electron rich.
When looking at the ammonia
molecule in the equation, we notice that the nitrogen atom has a lone pair of
electrons. Therefore, the ammonia molecule is
electron rich and will act as a Lewis base. During the reaction, ammonia will
donate a pair of electrons to the boron trifluoride molecule, forming a bond.
Now let’s discuss the boron
trifluoride molecule. In the boron trifluoride molecule,
the boron atom is electron deficient. We can describe the boron atom in
this molecule as electron deficient because the total number of bonding and
nonbonding electrons in its valence shell is less than eight. In fact, boron shares three pairs
of electrons with fluorine atoms. So it only has six electrons in its
valence shell. Therefore, it can accept another
pair of electrons to form a new covalent bond.
The boron trifluoride molecule will
act as a Lewis acid during the reaction and accept an electron pair from the ammonia
molecule. A new bond forms between the
nitrogen atom and the boron atom. The product formed when a Lewis
acid reacts with a Lewis base is known as an adduct.
Let’s bring the answer choices back
up on the screen to select the correct answer. In conclusion, the empty box that
should be filled with the Lewis acid term is box 2. Therefore, the correct answer is
answer choice (B).
