Question

Which are Lewis acids and which are Lewis bases? (a) \(\mathrm{Na}^{+}\) (b) \(\mathrm{NH}_{3}\) (c) CN" (d) BF \(_{3}\)

Short answer

Lewis Acids: \( \mathrm{Na}^{+} \) and \( \mathrm{BF}_{3} \); Lewis Bases: \( \mathrm{NH}_{3} \) and \( \mathrm{CN}^{-} \)

Step-by-step solution

Define Lewis Acids and Bases

Lewis acids are species that can accept a pair of electrons, whereas Lewis bases are species that can donate a pair of electrons.

Analyze \(\text{Na}^{+}\)

The sodium ion \( \mathrm{Na}^{+} \) lacks electrons and can accept a pair of electrons, making it a Lewis acid.

Analyze \( \mathrm{NH}_{3} \)

Ammonia \( \mathrm{NH}_{3} \) has a lone pair of electrons on the nitrogen atom, which it can donate, making it a Lewis base.

Analyze \( \mathrm{CN}^{-} \)

The cyanide ion \( \mathrm{CN}^{-} \) has extra electrons that can be donated, making it a Lewis base.

Analyze \( \mathrm{BF}_{3} \)

Boron trifluoride \( \mathrm{BF}_{3} \) has an incomplete octet and can accept a pair of electrons, making it a Lewis acid.

Key concepts

electron pair acceptor

In chemistry, an electron pair acceptor is a species that can accept a pair of electrons from another molecule or ion. This concept is crucial in defining Lewis acids. To be an electron pair acceptor, a molecule generally needs to have an electron-deficient area.

For example, the sodium ion \( \text{Na}^{+} \) lacks electrons (having a positive charge) and can accept a pair of electrons, making it a Lewis acid. Boron trifluoride \( \text{BF}_{3} \) has an incomplete octet and can accept a pair of electrons as well. Therefore, \( \text{BF}_{3} \) is also classified as a Lewis acid.

Remember, being an electron pair acceptor aligns closely with an atom's or molecule's need to fulfill its electronic configuration, often aiming to reach a stable arrangement.

electron pair donor

An electron pair donor is a species capable of providing a pair of electrons to another molecule or ion. This characteristic defines Lewis bases. Essentially, for a molecule to be an electron pair donor, it must possess a lone pair of electrons.

For instance, ammonia \( \text{NH}_{3} \) has a lone pair of electrons on the nitrogen atom. It can donate this pair to form a bond, making it a Lewis base. Similarly, the cyanide ion \( \text{CN}^{-} \) has available pairs of electrons that it can donate, classifying it as a Lewis base.

Being a donor usually means the molecule has electrons it can share, assisting in completing the electronic configuration of an electron-deficient species.

Lewis theory of acids and bases

The Lewis theory of acids and bases broadens the definitions we traditionally know. While Brønsted-Lowry theory focuses on proton donors and acceptors, Lewis theory shifts attention to electron pairs.

According to Lewis theory:

• A Lewis acid is any species that can accept a pair of electrons.
• A Lewis base is any species that can donate a pair of electrons.

This theory allows us to understand reactions that don't involve proton transfers but rather focus on electron pair exchanges.

For example, when \( \text{BF}_{3} \) bonds with \( \text{NH}_{3} \), \( \text{BF}_{3} \) accepts an electron pair from \( \text{NH}_{3} \), without involving any protons.

incomplete octet

The term 'incomplete octet' refers to molecules that do not have eight electrons in their valence shell. This is common with certain elements, especially those in the third period of the periodic table and beyond.

In Lewis theory, species with an incomplete octet are often electron pair acceptors, as they aim to achieve a more stable, full octet by accepting electron pairs.

For instance, boron trifluoride (BF\text{\textsubscript{3}}) has only six electrons in its valence shell around boron. This lack of a full octet makes \( \text{BF}_{3} \) highly reactive and eager to accept electron pairs from donors like \( \text{NH}_{3} \). Thus, \( \text{BF}_{3} \) is classified as a Lewis acid due to its incomplete octet.