MO Theory: Homonuclear Diatomic Molecules - Online Tutor, Practice Problems & Exam Prep

Understanding molecular orbital theory is essential when studying homonuclear diatomic molecules, which consist of two identical atoms, such as N₂ or F₂. The molecular orbital diagrams differ between elements based on their period in the periodic table. For period 1 elements like hydrogen and helium, valence electrons reside in the 1s orbital, leading to the formation of bonding (σ_1s) and antibonding (σ*_1s) molecular orbitals. As we move to period 2 elements, from lithium to nitrogen, the 2s and 2p orbitals come into play, creating a more complex arrangement with bonding (σ_2s, π_2p) and antibonding (σ*_2s, π*_2p) molecular orbitals. However, for oxygen to neon, due to increased electronegativity and decreased atomic size, the order of σ_2p and π_2p orbitals is reversed, impacting the molecular orbital structures. This concept is crucial in predicting the behavior and bonding of molecules.

MO Theory: Homonuclear Diatomic Molecules Concept 1

Now recall that a whole nuclear diatomic molecule is composed of two identical elements bonded together. Good example of this is N₂ or F₂. Now as a result of increase in electronegativity and a decrease in atomic size, the order of Σ2P and Π2P will be reversed for oxygen to neon.

So if we take a look here at molecular orbital diagrams in this first one this correlates to only H₂ to HE₂. So hydrogen and diatomic helium which is not a normal structure. And remember we're looking at the valence electrons for these elements. In hydrogen and helium their valence electrons are found into 1S orbitals or in. One of the periodic table. So here we'd have for instance, this could be the atomic orbital of hydrogen or helium and the other hydrogen or helium. We would fill in the atomic orbitals and then distribute those electrons into our molecular orbital sphere. Remember Σ1S represents our bonding molecular orbital and Σ*1S represents our antibody molecular orbital.

Now if we move to the next one, this correlates to diatomic lithium all the way to diatomic nitrogen. Here it gets more complex because now we're dealing with period two elements. So we'd start off with 2S and we move our way up to 2P. Now here with the 2S orbitals atomic orbitals, we fill in first our bonding molecular orbital which is Σ2S and then start filling in our Σ*2S which represents our antibody molecular orbital. When we get to 2P gets more complex as the atomic orbitals continue to pull their electrons together to create new molecular orbitals. Here the order forward would be Π2P Σ2P. Then it'll go Π*2P which is an antibody molecular orbital to Σ*2P another antibody molecular orbital. Again, we always fill from lowest molecular orbital up.

Now again, because of an increase in electronegativity and a decrease in atomic size, we're going to have a slight change when it comes to diatomic oxygen to diatomic neon. If we look the change happens here they flip. So now my Π2P moves up and my Σ2P moved down South. Here again, oxygen 2 neon. There's still period two elements. So we're starting out with two S again and we just start pulling together our atomic orbital electrons and dumping them into our molecular orbitals and start filling it up as we move up.

So just keep in mind these are the different types of molecular orbitals that can exist based on what type of. Element. You're dealing with 1-2 or three. All right. So keep this in mind as we start doing more and more questions dealing with homonuclear diatomic molecules.

MO Theory: Homonuclear Diatomic Molecules Example 1

Using a molecular orbital diagram or MO diagram, wipe the electron configuration for the diaphoring anion F₂^-, 1. All right, so let's follow the steps. Step one, we need to determine the number of valence electrons that this particular anion possesses. Fluorine is in Group 7A, so it has seven valence electrons, and there are two of them, so that's times 2. So that's 14 valence electrons. -1 means we've gained an additional electron, so in total we have 15 valence electrons.

Step 2. We're going to construct a molecular orbital diagram based on location of the valence electrons. Remember, if we're dealing with period one elements, that means the electrons start in 1S. If we're dealing with period two elements, they start with 2S and period three elements start with 3S. Now we're going to use the molecular orbital diagrams we have here. Now remember, we're dealing with two fluorine, two fluorines involved because it's F₂ and it's -1. So remember we're going to say it has some valence electrons, so that'd be up, down, up, down, up, up, up. So that's fine. 6/7 up, up, up, and then there. But remember, we have one additional electron. You can put it in on either one, so we'll put it here for that additional electron.

Now we would just pull those electrons together into our molecular orbitals. So here we'd have U down, U down, U down. So that's 2 4 6 7 8 9 10 11 12 13 14 15. So this would represent the molecular orbital diagram for the diflora fluorine anion. Now here we've filled in the total number of valence electrons for each element into the molecular orbitals. And now to fill in our electron configuration, it's a little bit different. So the way we do it is we show each one of our molecular orbitals as individual things here within parentheses, and up here we put the number of electrons found in each.

So if we look, we have two electrons within the Sigma 2S orbital. 2 electrons in Sigma star 2S², electrons within Sigma 2P⁴. Total electrons within Pi 2P⁴, total electrons within π star 2P, and then finally one electron within our Sigma star two. So those are the numbers we place down here. So we're going to come back down here and we're just going to fill them in, right? So what do we say here? We said we had two electrons here, 2 electrons here, 2 electrons here four, and then we had if we come back up here we had four in PI2P4 in π star 2P and one in Sigma star 2P. So come back down here, make sure we fill them in correctly, so four. And then finally here we have one O.

This would represent the filled in molecular orbital electron configuration for our DI fluorine anion. So again, this is the approach you would take in terms of filling out the molecular orbital for your designated diatomic molecule or ion, and then you would set up your molecule orbital electron configuration in this fashion and fill in the number of electrons found within each particular molecular orbit.

Determine the number of electrons found in the π_2p orbitals for the dioxygen dication, O₂²⁺. 

• A. 2
• B. 0
• C. 4
• D. 3

Using a MO diagram, write the electron configuration for the P₂ molecule

• A. (σ_2s)²(σ*_2s)²(π_2p)⁴(σ_2p)²
• B. (σ_2s)²(σ*_2s)²(π_2p)²(σ_2p)²(π*_2p)²
• C. (σ_3s)²(σ*_3s)²(π_3p)⁴(σ_3p)²
• D. (σ_3s)²(σ*_3s)²(π_3p)²(σ_3p)²(π*_3p)²