Explain Lewis acid Cation, Alcl₃ Base Amonia...

(a) Why are cations Lewis acids?

 * Lewis acids are defined as species that can accept an electron pair.

 * Cations are ions with a positive charge. This positive charge indicates that they have a deficiency of electrons compared to the neutral atom.

 * To achieve stability, cations tend to accept electrons to neutralize their positive charge.

 * Therefore, since cations have a tendency to accept electron pairs, they fit the definition of Lewis acids.

Example:  H+  +  :NH3  ==>  [H3NH]+

*  H+ (cation) acts as a Lewis acid by accepting the lone pair from :NH3.

(b) Why is ammonia (NH3) a Lewis base?

 * Lewis bases are defined as species that can donate an electron pair.

 * Ammonia (NH3) has a lone pair of electrons on the nitrogen atom.

 * This lone pair is available for donation to another species to form a coordinate bond.

 * Therefore, since ammonia has the ability to donate an electron pair, it fits the definition of a Lewis base.

Example:  H+  +  :NH3  ==>  [H3NH]+

* :NH3 (ammonia) acts as a Lewis base by donating its lone pair to H+.

(c) Why is AlCl3 a Lewis acid?

 * Aluminum Chloride (AlCl3) is a Lewis acid, even though it doesn't have an obvious positive charge like a cation.

 * The central aluminum (Al) atom in AlCl3 has only six electrons in its valence shell (3 from its bonds with the three chlorine atoms). It is two electrons short of a stable octet.

 * This electron deficiency makes AlCl3 strongly electron-pair accepting. It readily reacts with Lewis bases to form coordinate bonds and achieve a stable octet configuration.

 * Therefore, due to its ability to accept an electron pair to complete its octet, AlCl3 is a Lewis acid.

Example:  AlCl3  +  :NH3  ==>  Cl3 Al:NH3

* AlCl3 acts as a Lewis acid by accepting the lone pair from :NH3.

In summary, the definitions of Lewis acids and bases focus on the acceptance and donation of electron pairs, respectively. Cations, ammonia, and AlCl3 fit these definitions due to their charge or structure leading to electron pair acceptance or donation.