Why Does Telluric Acid Not Follow the Expected Pattern?

[END]

According to Wikipedia, the proper chemical formula for Telluric Acid is Te(OH)_6.

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 H_2SeO_4 is analogous to Sulphuric acid H_2SO_4. If this is the case, why is it that Telluric Acid differs from this pattern? I would expect its chemical formula to be H_2TeO_4, but apparently this represents Metatelluric acid which is the analogue of sulfuric acid and "is unknown." (Wikipedia)

Why is this the case?

Thank you!

 

[jim mcnamara]

from http://tellurium.atomistry.com/telluric_acids.html

Tellurium trioxide gives rise to several hydration products which can all be regarded as telluric acids, but which are more conveniently considered as products of the further hydration of telluric acid, H2TeO4, the compound H2TeO4.2H2O (possibly an "ortho-" telluric acid, H6TeO6) being the most stable.

H2TeO4.2H2O can be written as Te(OH)6 making the water of hydration look like hydroxyl groups.

 

[END]

Thanks, Jim.

So would $H_2TeO_4\cdot2H_2O$ then be considered a hydrate due to the presence of water molecules within it?

In addition, how are we supposed to know that telluric acid is a hydrate? (i.e., that it has a $nH_2O$)

 

[Borek]

I feel like you are trying to describe reality with simplified formulas, which won't work.

Hydrates often have well defined water molecules in the crystal lattice (and I believe that's the way you are trying to picture the telluric acid). But that's not the case here. We call observed substances "hydrates" because they differ by a water molecule, but they are actually different (although related) compounds. This is quite common - similar thing happens for example with sodium bisulfite and sodium metabisulfite.

 

[snorkack]

According to Wikipedia, the proper chemical formula for Telluric Acid is Te(OH)_6.

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 H_2SeO_4 is analogous to Sulphuric acid H_2SO_4. If this is the case, why is it that Telluric Acid differs from this pattern?

In Group IV, carbonic acid is H_2CO_3, but silicic acid is H_4SiO_4 - metasilicic acid H_2SiO_3 is polymeric.
In Group V, nitric acid is HNO_3, but phosphoric acid is H_3PO_4 - metaphosphoric acid HPO_3 is polymeric.
The reason both cases being that the bigger central atom prefers bonding to more oxygens.

Same reasoning applies to tellurium and iodine - Te(OH)_6 has Te bonded to 6 equal oxygens, whereas metatelluric acids are polymeric structures where some oxygens are bonded to two tellurium atoms.