Which of the following is classified as a soft acid?

In the context of acid-base chemistry, particularly in the Lewis theory of acids and bases, a "soft acid" refers to a cation that is more polarizable and has a preference for forming bonds with soft bases, which are typically larger, more polarizable anions.

Sodium ion (Na+) is classified as a soft acid because it has a low charge density due to its single positive charge and larger ionic radius compared to the other options. This allows it to engage more easily with soft bases, as its cationic nature facilitates the stabilization of electron-rich sites.

On the other hand, magnesium (Mg2+) and aluminum (Al3+) are considered hard acids. The higher charge density of Mg2+ and Al3+ (with their more significant positive charges and smaller radii) makes them less likely to interact with soft bases and instead favors interactions with hard bases. Potassium ion (K+) is also a soft acid in some contexts, but it is larger and less commonly referred to as a soft acid compared to sodium.

Thus, sodium ion’s characteristics align best with those of soft acids, making it the correct answer in this classification.