# Why is Ag+ a soft acid?

A soft species is typically large, polarizable and has a low charge (magnitude). The silver cation is not highly charged (this supports softness) however, it is anomalously small due to high nuclear charge and poor shielding by the d-orbitals. Consequently, shouldn't it also be less polarizable?

It does have a tendency to form bonds that are significantly covalent in nature which backs up the assertion that $\ce{Ag^+}$ is soft; however, I don't understand why.

• Well, it's still big enough... – Mithoron Feb 12 '15 at 13:28

## 1 Answer

According to HSAB theory, Soft Acids have:

large atomic/ionic radius;

low or zero oxidation state bonding;

high polarizability;

low electronegativity.

The ion $\ce{Ag+}$ satisfies all the aforementioned properties. I don't agree with you when you say it is anomalously small due to high nuclear charge and poor shielding by the d-orbitals. The effective nuclear charge has nothing to do here.

According to http://www.webelements.com/silver/atom_sizes.html: The radius of this ion is $114\ \mathrm{pm}$ when it's in 4-coordinate, tetrahedral geometry and it's $116\ \mathrm{pm}$ when it's 4-coordinate, square-planar geometry. Please just compare these radii with the radius of $\ce{Al^3+}$ ion : $53.5\ \mathrm{pm}$ which is considered as a hard acid. I hope it's clear now!

• Hi, sorry for reopening this topic, but the K+ ion is considered hard and its radius according to your source is 152pm, which is larger than the silver+ ion, so what causes K+ to be hard? – phi2k Mar 20 '18 at 15:09