Video Transcript
Which of the following species is
not a Lewis acid? (A) H+, (B) Mg2+, (C) NH3, (D)
AlCl3, or (E) BF3.
A Lewis acid is an electron pair
acceptor. If a species can accept a pair of
electrons, then it is likely to be electron deficient. The question asks us, which of the
following species is not a Lewis acid? So we need to work out which of the
options from (A) to (E) cannot accept a pair of electrons. Option (A) is H+. Capital H is the chemical symbol
for hydrogen. If we find hydrogen in the periodic
table, then we can see that its proton number, or atomic number, is one. Therefore, a hydrogen atom contains
one electron.
A hydrogen ion has a one plus
charge. This means that the ion will have
one fewer electron than the atom. So, a hydrogen ion contains no
electrons; it is just a proton. But since the first electron shell
can contain a maximum of two electrons, an H+ ion can accept an electron pair. In doing so, it would fill the
first electron shell. An example of a hydrogen ion
accepting an electron pair is the donation of an electron pair on a water molecule
to an H+ ion. This produces H3O+, which is known
as a hydronium ion. Since an H+ ion can accept an
electron pair, it is a Lewis acid. So, option (A), H+, is not the
answer to this question.
Option (B) is Mg2+. Like in option (A), this species
also has a positive charge, which is indicative of the species being electron
deficient. So we can already tell that Mg2+ is
likely to be a Lewis acid. Mg is the chemical symbol for
magnesium. Magnesium is in group two of the
periodic table, so magnesium atoms have two outer shell electrons. The magnesium ion in the question
has a two plus charge. Therefore, it will have two fewer
electrons than the magnesium atom, so the Mg2+ ion can accept an electron pair. And in doing so, it will have the
same number of electrons as a magnesium atom. Since an Mg2+ ion can accept an
electron pair, it is a Lewis acid. Thus, option (B) Mg2+ is not the
answer to this question.
Option (C) is NH3. NH3 is ammonia. Ammonia contains a lone pair of
electrons, where a lone pair is defined as a pair of valence, or outer shell,
electrons not shared in a covalent bond. The lone pair suggests that ammonia
is electron rich, not electron deficient, like a Lewis acid tends to be. Since the lone pair of electrons is
not shared in a covalent bond, the lone pair will be available to donate to other
species. Therefore, it’s much more likely
that ammonia acts as a Lewis base, where a Lewis base is defined as an electron pair
donor. This makes it very unlikely that
ammonia is an electron pair acceptor. Thus, it is most likely that
ammonia is not a Lewis acid, in which case option (C), NH3, is the answer to this
question. But to confirm, let’s look at
options (D) and (E).
(D) and (E) are similar species, so
let’s look at them together. Option (D) contains Al, aluminum,
and option (E) contains B, boron. Both of these elements can be found
in group 13 of the periodic table. Therefore, they both contain three
outer shell electrons. In options (D) and (E), both of
these atoms have formed three bonds. Aluminum bonds to three chlorine
atoms, forming aluminum chloride, and boron has bonded to three fluorine atoms,
forming boron trifluoride, or trifluoroborane. All three of the outer shell
electrons in aluminum and boron are involved in bonding.
The second electron for each bond
comes from the chlorine and fluorine atoms, respectively. For these species, aluminum and
boron contain six outer shell electrons, but the octet rule states that atoms tend
to bond so that they have eight outer shell electrons. This means that both AlCl3 and BF3
can accept two electrons, or an electron pair, so that they satisfy the octet
rule. As AlCl3 and BF3 can both accept an
electron pair, they are both Lewis acids. This means that neither option (D)
nor (E) can be the answer to this question.
Therefore, the answer to the
question “Which of the following species is not a Lewis acid?” is (C) NH3.
