Suggest a plausible structure for the silicate anion in the mineral thortveitite, Sc₂Si₂O₇.

Video transcript

Welcome back everyone. Let's take a look at our next problem. What is the possible tetrahedron structure for the silicate anion in gadolinium pyro silicate? And its chemical formula is GD two SI 207. So let's think about what a tetrahedron structure for a silicon anion is. We know that the basic unit of a silicon anion is si 04 with a charge of four minus silicon atom surrounded by four oxygen atoms, each of those oxygens with a formal charge of negative one. And so we can draw that structure as a tetrahedron where are silicon atom is in the center, each corner represents an oxygen atom and each unshared corner has a formal charge of negative one. Since the oxygen there will have three lone pairs and one shared electron giving in that formal charge of negative one. But we see in our compound here, we have two silicons and seven oxygens. So we know that one of those oxygens must be shared. And this is quite common in silicate compounds or silicon anions. And if we just look at, take a quick look at the Lewis structure, we can see why we have our two silicon atoms and they share an oxygen as a bridge between them, then each of them has three terminal oxygens. And we know those terminal oxygens will each have three lone pairs. And that formal negative one charge, when we look at that bridging oxygen, it is having those two covalent bonds with the silicon atoms and therefore just has two lone pairs giving it a formal charge of zero. So if we look at this anion as a whole, this si 07, we'd expect it to show a charge of six minus as its ion. And we can double check that by if we calculate our using our usual oxidation states of silicon and oxygen. So for silicon, that would be plus four times or multiplied by two silicon atoms would be plus eight. While for oxygen, we have its usual oxidation number of negative two multiplied by seven equals negative 14. And that would lead us to ex expect an overall charge of negative six, which corresponds with what we'd expect from the proposed Lewis structure in our compound gadolinium is a transition metal. So it can have different ionization states. So that's why we look at our silicon anion to calculate that expected ionization char or charge of the anion there. So now when we go to think about what a possible tetrahedron structure would look like in this case, well, we know we have two silicon atoms. So therefore, we must have two tetrahedrons we have seven oxygen atoms and therefore must have seven corners to our structure. And our charge on the anion is negative six and therefore, we must have six uns shared corners. So since we have seven corners expected six uns shared, we would expect our two tetrahedrons to share one corner, which makes sense since we expect to have its bridging oxygen atom. So let's draw that tetrahedron structure. So put two of them up there. And indeed two tetrahedrons touching at one corner would be a possible tetrahedron structure. We have two tetrahedrons, seven corners total six of them un shared. So that would be our answer choice. It's only one possible structure because one of those tetrahedrons could be twisted with respect to the other. But this is a possible structure right here filling all our conditions for the silicon anion and gadolinium p silicate. See you in the next video.