Describe the molecular geometry of:

(b) AsH₃

Video transcript

All right. Hi everyone. So for this question, let's go ahead and draw the structure or a structure of ph three and describe its molecular geometry. So Ph three is also referred to as phosphine. Now there are two ways of going about this question starting off with simply going from scratch from the very beginning, right. So in this method, you take your molecular formula which is ph three and you find the number of valence electrons that each atom is going to contribute to the Lewis structure starting off with phosphorus. Now phosphorus, if you recall is in group five a of the periodic table, which means that it has five valence electrons in Haugen on the other hand, has one valence electron. However, there are three atoms of hydrogen present in this molecule. So I will multiply the one valence electron by three to find that there are three more valence electrons that the hydrogen atoms are contributing. So when I add together my five valent electrons from phosphorus and the three from my hydrogen atoms, I get that there are a total of eight valence electrons to distribute in my Lewis structure. So now in my actual Lewis structure recall that hydrogen is only capable of making one covalent bond. Therefore, phosphorus is going to be in the center. And I'm going to draw three different bonds from phosphorus to each hydrogen atom. Now, three covalent bonds multiplied by two electrons. Each gives me a total of six electrons that I have spent so far or that I have used. So now the remaining two electrons that I have left are going to go on phosphorus as an extra loan pair of electrons. And so here is the lowest structure of phosphate. Now, the other method that's arguably faster would be to recognize that there are similarities between the structures of phosphine and the structure of ammonia or NH three, right. And the reason for this is because both nitrogen and phosphorus belong in the same group of the periodic table and therefore have the same bond preference, meaning both phosphorus and nitrogen prefer to have three covalent bonds and one lone pair of electrons. So by recognizing the similarity, you can also find the lowest structure of phosphine by taking the structure of ammonia and simply replacing the nitrogen in the center with phosphorus. However, either way will lead you to the same answer, right. So now with regard to the geometry, when it comes to the geometry, we have one lone pair of electrons as well as three covalent bonds to other atoms, which gives us a total of four electron domains. Now, when we have four electron domains. One of them being a lone pair of electrons recall that that corresponds to trigonal petal geometry. And there you have it. So here is the Lewis structure of phosphine found in two different ways with a geometry described as trigonal parameter. And with that being said, thank you so very much for watching. And I hope you found this helpful.