The Mystery of Silver

My chemistry teacher told us at GCSE that at A Level, we would learn why silver (Ag) only has one oxidation state - Ag⁺. Yet we never did - instead, we got a sheet which said this was the case, and that was final. This has somewhat bothered me, especially as silver isn't even that common an element at A Level - sure, it comes up here and there, but otherwise it's mostly relegated to a few sub-chapters.

In hindsight, the reason why silver only has one oxidation state is incredibly obvious, and it's a similar reason to why zinc only has one common oxidation state - Zn²⁺. 

Silver, like zinc, is a d-block element, which means its valence electrons are present in its d-orbital, specifically the 4d orbital in the case of silver. Its electron configuration is [Kr] 4d¹⁰5s¹ - it has a full d-orbital, which is all well and good. However, there's an additional electron present in the 5s orbital, which is at a higher energy level to the rest of the d-block elements - this is what ultimately makes a silver atom unstable, and susceptible to ionisation. When silver ionises, it loses the 5s electron first, as it's from a higher energy level, to form an Ag⁺ ion. As the d-orbital is full, any additional ionisation energies for silver will be much higher, and so Ag⁺ is the dominant ionic state of silver.

That should be it - however, there are some caveats to this:

• Silver is below copper on the periodic table, so copper will have a similar electron configuration to silver (Cu: [Ar] 3d¹⁰4s¹) and thus both ought to have similar chemical properties. And yet copper is able to form various different oxidation states beyond +1. 

The reason for this is that silver happens to have a lower first ionisation energy to copper, but not significantly so that copper is far less willing to lose its 4s electron than silver is to give up its 5s electron. Come the second ionisation energy, however, and this trend reverses - this is why Cu²⁺ is far more common than Ag²⁺, for example; Cu²⁺ is in fact the most common copper ion.

Ionisation energies	Copper IE (eV)	Silver IE (eV)	Gold IE (eV)
First	7.726	7.557	9.226
Second	20.29	21.48	20.20

Therefore, silver is restricted to the +1 oxidation state world. 

You will have noticed that I also added gold's data - gold (Au) is directly below silver in the periodic table (ironically a reverse of Olympic podiums) so gold should be similar to silver chemically-speaking. And yet gold actually does the opposite of silver - its first ionisation energy is massive in comparison to copper or silver, and yet its second is the smallest out of the three. 

However, it's worth pointing out that Au²⁺ is far, far, far less common than Au³⁺ is, with that being gold's predominant ionic state. This is actually not exactly unusual, since many of the elements in the same period as gold - Period 6 - like all the Lanthanides, iridium, thallium, and bismuth also having +3 as a common oxidation state. (Worth noting the Lanthanides are an entirely separate can of worms, though.) Yet the scope of this blogpost can't possibly contain gold's properties as well, they would deserve a separate writeup. 

In relation to silver, it looks like +1 is as far as one can go. Although...

• That's not entirely the case. Silver can form oxidation states greater than +1, it's just that more energy will need to be input to allow for Ag⁺ to lose another electron - yet this is one electronvolt more than copper, which on the atomic scale is significant.

But even then, Ag⁺ is the main ion present in various commonly-known silver compounds. One of them, silver nitrate (AgNO₃), is used in ions testing to see whether any halide ions are present, as I discussed in this blogpost. [Ag(NH₃)₂]⁺, meanwhile, is an ion present in Tollens reagent, a chemical used to test for the presence of an aldehyde - if there is an aldehyde present, then a silver mirror forms, as a result of a redox reaction, where Ag⁺ is reduced to Ag (the NH₃ ligands aren't involved in the reaction itself).

A positive Tollens reagent test

So, to answer the question which my teacher never answered:

• Ag only has one common oxidation state as any additional ionisation energies beyond the first one are far higher than those of neighbouring elements.
• That doesn't mean Ag only has one oxidation state - it can form others, it just requires more energy to be put into the system.
• That doesn't change the fact, though, that Ag⁺ is simply far more ubiquitous amongst chemicals commonly used today.

Ionisation ion energy data was taken from this website: https://physics.nist.gov/PhysRefData/ASD/ionEnergy.html

Image of silver clasps courtesy of W.carter from Wikimedia Commons, Image of Tollens reagent test courtesy of Tmv23 on Wikimedia Commons.