Re: can you tell me the second electron affinity of oxygen please.

The electron affinity is a measure of how tightly a negative ion holds onto its extra electron -- or 
of how strongly a neutral atom attracts an extra electron. They are both the same thing, even if 
they might not sound like it.

When we talk about ions, we are meaning atoms that have lost or gained electrons so that they are 
positively or negatively charged. A sodium atom, for example, can be fairly readily persuaded to 
give up one of its electrons and become a positively charged Na+ ion; A calcium atom often gives 
up two electrons to become a Ca2+ ion. A chlorine atom, on the other hand can often pick up an 
extra electron and become a Cl- ion. With group 6 elements, we often think of doubly charged 
negative ions like O2- and S2-. It all makes a very neat and symmetrical picture, and you will find 
all chemistry textbooks talking about ions in this sort of way.

You have been looking for the second electron affinity of oxygen: the energy it takes to remove an 
electron from O2- to make O-. And you have failed to find it! There is a good reason. An isolated 
oxygen ion, O-, has no affinity for a second extra electron. If a second electron comes close to O-, 
the two negative charges will simply repel each other, and the electron will be pushed away! I 
cannot tell you the second electron affinity of oxygen, because oxygen does not have any affinity 
for that second electron!

That will sound like heresy to you, and probably to your chemistry teacher as well! Oxide ions, 
O2-, are familiar species that chemists talk about all the same. Calcium oxide has a crystal 
structure made up of Ca2+ and O2- that is absolutely identical with that of sodium chloride, made 
up of Na+ and Cl-. Well, yes, what you have heard before and read in textbooks is not all lies. 
Oxide ions, O2-, can and do exist, but only in solids and liquids where there are extra positive 
charges around to help the two negative charges in the O2- ion to stick together. It is a fair thing 
to talk about O2- in crystals, or in molten metals and slags, or in some solutions. Not in water 
solutions, because

O2-  +  H2O  -->   2 OH-

But in solids, melts or solutions the "electron affinity" is a quantity that would vary greatly 
depending on the exact context. Real electron affinities, like ionization potentials, are only defined 
for isolated atoms or molecules in the gas phase. And isolated O- ions in the gas phase simply do 
not attract another electron -- they repel it!

I am afraid I cannot easily give you a reference for this answer: I am not aware of any discussion of 
it outside very specialized journals. If you do a web-search you will find one or two sets of notes 
where an authority has got around the problem by quoting a value with the opposite sign -- how 
much energy do you have to provide to force a second extra electron onto an isolated O- ion. To 
my thinking that is a most peculiar notion of an "affinity".
(This page is an example. A value of 844 kJ/mol, with the wrong sign, is quoted.)