a. Why is the SO₃ molecule trigonal planar but the SO₃^2– ion is trigonal pyramidal?

Video transcript

Welcome back everyone. Our next problem says the molecule seo three exhibits a trigonal planar geometry. While the ion seo three to minus exhibits a trigonal parameter geometry explain why. So let's think about the structures of these two starting with the Lewis dot structure. So we have to think about how many valence electrons we have. Well, we know that selenium, we can look it up on the period oct se is in group six A and therefore has six valence electrons. While oxygen, there are three oxygens multiplied by its six valence electrons in that same group. So we have six plus 1824 valence electrons to account for. And we know that selenium is less electron than oxygen since it's down below it in its bro in its group. So we'd expect selenium to be the central atom. We have three oxygens. So let's just put them around ar selenium. We have 24 valence electrons. Now with three bonds, six of our electrons are in bonds. So subtract six bonding electrons and we're left with 18 to account for well, 18 electrons, three oxygens, we can distribute them as three lone pairs each which oxygen is pretty comfortable with. So put these lone pairs around our structure, we've accounted for all of our electrons. There's no lone pairs. So selenium has three bonds, no long pairs. So what kind of orbitals does selenium need to inform these three bonds? Well, it needs one S and two P orbitals. So it's sp two hybridized and with no long pairs that will give you a trigonal planar geometry trying to spell trigonal here, trigonal planar geometry. So we've explained our neutral molecule. Now, let's look at our ion, well, number of electrons, we had 24 valence electrons from the basic molecule works the neutral molecule, but we have an ion with a two minus charge. So we know we have two more electrons. So we have 26 valence electrons to account for. So lets write our selenium in the center, draw oxygens around it, three oxygens, three bonds. So we are going to subtract our six bonding electrons and that will leave us with 20 electrons. So if we put those three lone pairs on every oxygen as we did in our previous molecule, we still have two electrons left over. So let's pop them on selenium as a lone pair. However, this gives selenium 12345 electrons around it, it has six valence electrons. So it's not gonna like that configuration. But if we move one pair of electrons from one of our oxygens and form a double bond with selenium, selenium is capable of having expanded octet because it's one of the in one of the higher rows of the periodic table. So let's draw the structure that way. So now one of our oxygens has a double bond to selenium and just two lone pairs. Now selenium has 123456 around it and a formal charge of zero, it likes that configuration a little better note that this is a resonance structure. You could put the double bond on any of these oxygens. So we'll just note resonance structure here. But let's think about the geometry, we have three bonds plus one lone pair. So the orbitals that the linear needs to have to have these around it are one S and three P orbitals to accommodate that lone pair. So it's sp three hybridized, well, sp three hybrid orbitals are tetrahedral. But because we have a lone pair, we wouldn't draw that in our structure. So that leaves us with selenium, the lone pair up here and then our oxygens coming down to form this parameter shape. So rather than tetrahedral, since we do have a lone pair taking up one of the spots, and again, this will be a resonance structure, we have trigonal parameter geometry. So you see that the difference in structure between these two can be explained by the fact that our ion with its negative two charge has two extra electrons that have to be accommodated on the selenium as a lone pair. So the presence of this lone pair is the reason why you have the trigonal planar structure for your neutral compound. And the trigonal parameter structure for your ion. See you in the next video.