According to Wikipedia, the proper chemical formula for Telluric Acid is [itex]Te(OH)_6[/itex]. From what I've learned in chemistry thus far, acids made from polyatomic ions within the same group follow a similar pattern. For example, Selenic Acid [itex]H_2SeO_4[/itex] is analogous to Sulphuric acid [itex]H_2SO_4[/itex]. If this is the case, why is it that Telluric Acid differs from this pattern? I would expect its chemical formula to be [itex]H_2TeO_4[/itex], but apparently this represents Metatelluric acid which is the analogue of sulfuric acid and "is unknown." (Wikipedia) Why is this the case? Thank you!