Why does chlorate seem to break the pattern of polyatomic ions?

[FysicsPhorums]

Hi, I was reviewing the polyatomic ions, and on the third row of the periodic table they seemed to follow a pattern going from left to right on the periodic table. For example:
Silicate = SiO₄^4-; phosphate = PO₄^3-; sulfate = SO₄^2-

For row 3 on the periodic table, it looks like each polyatomic has 4 oxygens, the charge increases by one as one moves to the right along the periodic table, and I think the oxidation numbers of the non-oxygen elements goes +4, +5, +6, respectively. However, chlorate is ClO₃^-, which seems to break the pattern, since it has only 3 oxygens instead of 4, and the oxidation number on chlorine of +5, instead of +7.

Is there any reason chlorate seems to "break" the pattern of the polyatomic ions so far, and any reason why for naming purposes, ClO₃^- is chlorate, instead of chlorite (like PO₃^3- is phosphite, for example)?

Thanks!

 

[samblohm]

Well actually there is a Perchlorate ion that is ClO4 that has a negative 1 charge so the pattern isn't broken. Chlorine is just very flexible in a sense because it can have anywhere between 1-4 oxygen atoms and still have a minus 1 charge.
ClO is Hypochlorite
ClO2 is Chlorite
ClO3 is Chlorate
ClO4 is Perchlorate
The naming has to do with the other form of the polyatomic ion can be.

 

[FysicsPhorums]

So is ClO₄^- called perchlorate simply because there is no ClO₅^-? I understand how perchlorate would be named relative to chlorate, and so on, but I'm trying to figure out why the naming for the other polyatomic ions I mentioned had 4 oxygens for the "ate" names. But for chlorate, the form with 3 oxygens carries the "ate" name. Thanks again for the help!

 

[samblohm]

In general, the most common oxyanion for an element will end in ate. You will find many trends in the periodic table, but you will also find many exceptions.

Some examples I can think of for ate's that have 3 oxygens are nitrate, bromate, iodate, carbonate (although that has a minus 2 charge)

And I don't think there is an element that has 5 different oxyanions.