The following pictures represent various silicate anions. Write the formula and charge of each anion.

Video transcript

Hi, everybody. Let's take a look at the next problem. It says, consider the following silicate anion. And we have a tetrahedral shorthand structure here with four tetrahedrons with sort of a diamond shape in the center. So they're touching at four different points. It says, what are its formula and charge? And we have four different options here as choices A through D A, its formula is SI 4012, 8 minus and its charge is negative eight B. Its formula is SI 4012, 8 plus and its charge is positive eight C. Its formula is SI 4016, 4 minus and its charge is negative four or D. Its formula is SI 4016, 4 plus and its charge is positive four. Well, first of all, without even looking at our structure, figuring anything out, our problem tells us it's an anion. So we can go ahead and eliminate choice B and choice D because they both say that there's a positive charge. And we know that can't be the case since this is an anion. So let's look at our structure, remembering the rules of these tetrahedral shorthand structures. Each tetrahedron is one si 04 subunit with a silicon atom in the center. And each corner representing an oxygen atom, the unshared corners represent oxygens that have a negative one formal charge. While the shared corners represent an oxygen that has bonds bridging two silicon atoms and therefore has no formal charge. So let's look at our structure and follow these rules and figure out our formula. So we have four tetrahedrons, four units. So that means our formula is going to have si 44 silicon atoms. Then we also have so four silicon atoms, let's look at our shared corners. So we have 1234 shared corners where four tetrahedrons are touching, they make sort of a ring. So we have four shared corners which represent four oxygen atoms with a charge equal to zero. I'm putting a line to that zero because I've got the letter O all over the place because of oxygen. Then I have, I use red for the unshared corners. I have 1234 unshared outside corners on my figure and then 5678 representing the four pyramid tips for lack of a better word. So a total of eight unshared corners. So that would represent eight oxygen atoms with a charge equal to negative one. So we have a total of 12 oxygen atoms. So we're going to have si 4012 and then we have four atoms of oxygen atoms with zero charge. And then eight oxygen atoms with a charge of a negative one. So the charge on our ion is going to be negative eight. So si 40 12 8 minus. So that will be our chemical formula. So when we match it up with our answer choice, that would be choice A because choice C says si 40 16 4 minus with a charge of negative four. So that's incorrect. So once again, the silicate anion were given choice A its formula is si 4 12 8 minus and its charge is negative eight. See you in the next video.