How could you figure out how many oxygens a polyatomic ion has?
This of course depends on the polyatomic ion you choose, but...
Let's consider a general polyatomic ion,
Suppose we wanted to look at the nitrate ion. I won't tell you the formula, and we'll figure out what its formula is.
Consider breaking down the word into the "nitr" stem and "-ate" suffix.
- "nitr" is the stem for "nitrogen", whose chemical symbol is
#"N"#. So, we have one of the elements figured out.
- The "-ate" suffix implies that the polyatomic ion has more than one oxygen. In fact, it must have at least three.
This is just a conclusion based on relating back to other known "-ate" polyatomic ions, such as chlorate (
#"ClO"_3^(-)#), perchlorate ( #"ClO"_4^(-)#), sulfate ( #"SO"_4^(2-)#), carbonate ( #"CO"_3^(2-)#), and so on. These all have at least three---but not necessarily three---oxygens.
Now, to have a clue as to how many oxygens makes sense, let's think about the valency of nitrogen.
- Its electron configuration is
#1s^2 color(green)(2s^2 2p^3)#, which means it has #\mathbf(5)#valence electrons.
- That means it can have a maximum oxidation state of
#\mathbf(""^(+5))#by losing all #5#.
Finally, recall that oxygen has a common oxidation state of
So, we really only have one or two possibilities for nitrate, where the total oxidation states are marked atop:
#stackrel(color(green)(5+))("N")stackrel(color(blue)(8-))("O"_4^(3-))}#Higher overall charge
#=>#generally more unstable
The first possibility is reasonable, given that it has the lower overall charge.
(The second is very unstable, though it's real.)