The following models represent the structures of binary

oxides of second- and third-row elements in their highest

oxidation states:

(a) Identify the non-oxygen atom in each case, and write the molecular formula for each oxide.

Video transcript

Alright. Hi everyone. So this question says that the following models represent the structures of oxides of 2nd and 3rd period elements in their highest possible oxidation states provide the atom that is not oxygen in each case and identify the molecular formula for each assume that there are no formal charges on any atom and the bonds shown can be double or triple bonds. So recall first and foremost that the oxidation state of oxygen in an oxide is going to be equal to negative two. No, the question mentions that all of the models represent molecules which means that there can only be covalent bonds between oxygen and the other atom, which means that there should not be any ionic oxides. Now, because of this metals are not likely to be involved in either of these compounds but not necessarily impossible, right? So let's start with part one. The compound in question has a total of two atoms, one of which is oxygen, meaning that there can only be one other atom that is not oxygen. So as mentioned previously, right, the oxidation state of oxygen is going to be equal to negative two, which means that in order to keep this compound neutral overall, the other atom must have an oxidation state of positive two to balance out the charges. Therefore, we have to look for group two A elements. Now, the problem with this is that the the elements in group two A are metals which would normally form ionic oxides. However, we do have an exception and that exception is beria and recall specifically that beryllium is capable of forming covalent bonds due to its size. Now, the other candidate in this case is magnesium, but bum is better at forming covalent bonds. Therefore, the non oxygen element must be beryllium and the molecular formula must be beo. So there's part one right, the non oxygen element is beryllium and the molecular formula of the oxide is be up. So now we can proceed with part two. Now, in part two, the molecule in question has 14 total atoms. Now four of which cannot be oxygen, meaning that there are 10 oxygen atoms present in part two. Now, if we go ahead and consider the model of part two, notice how there are three different types of atoms, right? One of which only has two bonds, which could be oxygen because oxygen does prefer two bonds, another of which only has one bond, which could also be oxygen because that one bond, it can be a double bond and one of them with four bonds. Now, the four bonds would not be possible for oxygen because recall that oxygen cannot have an expanded octet. So if we consider that mysterious element, recall that we previously defined that there's going to be one double bond between the element and oxygen. Whereas the remaining three must be single bonds, this means that the non oxygen element must have a total of five bonds to maintain the structure. Now, recalled five bonds provides a total of 10 electrons, meaning that there must be an expanded octet and not only does there have to be an expanded octet, right? There has to be a maximum oxidation state of positive five because five bonds are being formed. This means that the element in question has to come from period three because recall that period two elements cannot have expanded octets. Therefore, the element in question has to be phosphorus. So now the formula of the oxide will be using the number of atoms per element as the subscript, right. So in this case, there are four non oxygen atoms and therefore four atoms of phosphorus and 10 atoms of oxygen. Meaning that for part two, right, the non oxygen element is phosphorus and the formula is P 4010. So now let's go ahead and proceed with part three. Now part three has four atoms in total four atoms, oops in total one of which is not oxygen, meaning that three of the four atoms must be oxygen atoms. So first, let's consider the geometry of model three and because as you can see here, model three has trigonal planar geometry. Now, relative to the central atom, recall that there are three different ways in which trigonal planar geometry can be observed. One is if there are three single bonds and another is if there are three double bonds. Now, either way, right, the oxygen, excuse me, the atom in the center here must be the non oxygen atom because recall that oxygen normally prefers to have two bonds and two lone pairs, which means that it would not be able to expand its octet to accommodate more bonds than that. So the non oxygen atom must be in the center. And therefore all of the bonds shown between the central atom and each oxygen on the outside must be a double bond. So this means that the central non oxygen atom must be able to accommodate three double bonds to each oxygen, meaning that an expanded octave is required to accommodate the total of six bonds. And because it has six bonds, this also implies that there can be a maximum oxidation state of positive six. So in this case, a six a element is required that can expand its octet. Therefore, the element in question is sulfur. So using the number of atoms per element as the subscript, the molecular formula is so three and that is the answer to part three. So last but not least is part four. So if I scroll up to consider the structure of part four, there are a total of seven atoms, seven atoms, two of which are not oxygen. This means that there are going to be a total of five oxygen atoms present in the structure of part four. So here we can recognize that there are three different types of atoms, the one in the center here as two bonds, whereas the ones on the outside have only one bond and there are two others that have three bonds. So in this case, the oxygens can be the ones that have two bonds or only one bond because recall that the one bond can be a double bond. Now, oxygen cannot be the atoms here that contained three bonds because oxygen cannot have an expanded octet. So now the question is identifying the non oxygen atoms that have three bonds in the structure provided for part four. So here, as discussed previously, the two non oxygen elements can accommodate one bond as well as or rather to correct myself, the non oxygen elements can accommodate two double bonds as well as one single bond, meaning that it must accommodate a total of five bonds, meaning that we need an expanded octet once again. Now because we need an expanded octet and we have five bonds in this case, right, the maximum oxidation state is positive five, meaning that if we recall our discussion for part two, right, the element in question is going to be phosphorus. So by using the number of atoms per element as the subscript in the formula, our molecular formula becomes P 205. So that concludes part four as well as the rest of the question. So I will scroll up here to recap the answer for each part, right? For part one, the element that was in oxygen was bellum with a molecular formula of beo for part two, it was phosphorus with a molecular formula of P 410. For part three, it was sulfur for a formula of so three and for part four, it was phosphorus once again with a molecular formula of P 205. And with that being said, if you stuck around to the end, thank you so very much for watching. And I hope you found this helpful.