Question Video: Identifying the Species That Is Not a Lewis Acid in a Set of Chemical Formulas Chemistry

Which of the following species is not a Lewis acid? [A] H⁺ [B] Mg²⁺ [C] NH₃ [D] AlCl₃ [E] BF₃

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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.

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