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

In heteronuclear diatomic molecules, composed of two different elements, the molecular orbital energy levels are influenced by electronegativity, which increases towards the top right of the periodic table. The more electronegative element has atomic orbitals that are lower in energy, affecting the molecular orbital diagram used to represent these molecules. Electronegativity not only determines the positioning of elements in molecular orbital diagrams but also indicates that electrons are more tightly bound to the nucleus in elements with higher electronegativity. This concept is crucial when considering the electronic structure and chemical behavior of heteronuclear diatomic molecules in molecular orbital theory. Understanding the relationship between electronegativity and energy levels is essential for predicting molecular properties and reactivity.

MO Theory: Heteronuclear Diatomic Molecules Concept 1

Recall that a heteronuclear diatomic molecule is composed of two different elements bonded together, and we're going to say here that the less electronegative element determines which molecular orbital will be used. And recall that electronegativity increases as you move to the top right corner of the periodic table. Remember, fluorine is the most electronegative element with a value of 4.0.

Now we're going to say here that when it comes to displaying these molecular orbitals or these molecular orbital diagrams, the more electronegative element within the heteronuclear diatomic molecule possesses atomic orbitals that are lower in energy. So here, if we take a look, we have molecular orbital diagrams for hydrogen to helium, for lithium to nitrogen, and from oxygen to neon. If you've watched my videos on molecular orbital theory, you're kind of familiar with these displays.

Now if you notice, you can see here that this 2S atomic orbital is higher in energy than this one. This one is lower down. That must mean that let's say we're combining. We're comparing carbon to nitrogen. Carbon is less electronegative than nitrogen. Since nitrogen is more electronegative, it would be here on this side and carbon would be here on this side. And since nitrogen is more electronegative, that explains why its atomic orbitals are lower in energy.

See here. It's 2P orbitals are also lower in energy, carbons are higher in energy. And then here if we look at oxygen to neon, same thing. This 2S orbital atomic orbital is higher in energy than this one, meaning the left side is involved with an element that has less electronegative. So for example, maybe we have here oxygen versus fluorine.

OK, so keep in mind that electronegativity helps us determine which molecular orbital diagram to use and also it helps us to see the differences in energy for our atomic orbitals. Now just remember that the higher your electronegativity than the electrons will be closer to the nucleus, so they're going more tightly bound to the nucleus. What effect can this have on our shell number here?

So here our electrons are being pulled closer and closer to the nucleus. Since electronegativity is increasing, this will see a lot of the times with lower numbered shell elements such as flooring. So here we have a lower shell number. All right, so keep in mind these fundamental ideas when we're talking about heteronuclear diatomic molecules and MO theory.

MO Theory: Heteronuclear Diatomic Molecules Example 1

Construct the molecular orbital diagram for the carbon monoxide molecule. So Co Now here we're going to determine the right molecular orbital diagram. Carbon is less electronegative than oxygen, so we're going to use the molecular orbital diagram for carbon. So use this middle one here. Now remember that the more electronegative element has lower energy in terms of its atomic orbitals, so oxygen will represent this side and carbon will represent this side.

Now, in terms of number of valence electrons, we're going to say here that carbon is in Group fourane, so it has four valence electrons or valence electrons. So it'd be one, up, one down and then up up. Oxygen is in Group 6A, so it has six valence electrons, so one 2345 and then six. We now pull together our electrons in the atomic orbitals and put them into the molecular orbitals. So it'd be up, down, up, down.

Now we have a total of how many electrons here 6 electrons that we basically need to put into these remaining molecular orbitals. So it'd be up, up, down, down, and then we need two more up, down. So this will represent the filled in molecular orbital diagram. For carbon monoxide, we'd come down here and we'd fill it in. So we're going to say we have two of σ2S2 of σ2Sstar.

We see that we have 4-4 π2P and then finally we'd have two in the σ2P. So this would be all the electrons that we have to fill in this electron configuration for carbon monoxide. Now here these would have nothing in them, so you could in theory if you want just remove them from this because there D be 0 in here and zero in here. So we have our molecular orbital diagram above and this would be the completed electron configuration using that molecular orbital diagram for carbon monoxide.

Apply Molecular Orbital Theory to determine the MO orbital diagram for the CF⁺ ion.

Apply molecular orbital theory to determine the molecular orbital diagram for the carbon fluorine ion. Alright. So CF+. Carbon is in group 4A, so it has 4 valence electrons. Fluorine is in group 7A, so it has 7. That's 11 so far, but plus 1 means we've lost an electron. So, we're going to deduct 1 electron from this. Now we have 10 total valence electrons involved. Remember that the more electronegative element will have atomic orbitals that are lower in energy. On this side is fluorine and this side here is carbon. So here, we start filling in the atomic orbitals with 4 from carbon. Now, we're losing electrons here, so what's most important is filling out the molecular orbital section. We only have 6 more electrons that we can fill in. Next, we're going to pull them together into the molecular orbital diagrams. This would be up, down, up, down, that takes care of our 4. Then up here, we have a total of 6 electrons we need to distribute amongst here. This will be up, up, down, down, up, down. This represents the filled-in molecular orbital diagram for the CF+ ion.

Using a MO diagram, write the electron configuration for the BN molecule? 

• A. (σ_2s)²(σ*_2s)²(π_2p)⁴
• B. (σ_2s)²(σ*_2s)²(π_2p)⁴(σ_2p)²(π*_2p)⁴
• C. (σ_2s)²(σ*_2s)²(π_2p)²(σ_2p)²
• D. (σ_2s)2(σ*_2s)²(π_2p)²(σ_2p)¹(π*_2p)²(σ*_2p)¹