Question

[Chemistry] This is about the Activity Series, found here: http://www.files.chem.vt.edu/RVGS/ACT/notes/activity_series.html

"The last five metals (mercury - gold) are often found free in nature, their oxides decompose with mild heating, and they form oxides only indirectly."

What does it mean when they say the oxides only form indirectly? Could someone provide me with an example?

Answer 1

Here is the best possible answer I can come up with for you based on other pieces of various, slightly-related information that's been cobbled together:
Oxidation can be defined as the interaction of oxygen molecules and the molecules of other substances, or, more specifically, the loss of at least 1 electron when at least 2 substances interact. Oxides form when oxygen bonds to another element, resulting in the loss of an electron or electrons.
The term "indirect" implies that oxygen and the other substances either never actually directly interact, or that there is some sort of mediator that allows both of them to bond. If you look at the 3D structure of gold oxide, mercury oxide, etc, they form a matrix and are unstable relative to other oxides. This tells me that they don't bond easily, and when they do bond, the bond is fragile, which leads me to believe that the term "indirect" may mean that there is a mediator substance/molecule/particle involved, the substance doesn't truly oxidate which results in an unstable substance, or the oxygen and metal need to be united artificially in a lab setting.
I know there isn't a direct answer here (you may even say the answer is... INDIRECT! haha!) , but hopefully it's some food for thought and gets you going in the right direction.