Covalent Bonds – Quantum Numbers

00:01 So I talked earlier on about the importance of having a full outer shell in order to achieve stability. And this applies to all elements: not just those in the first three groups and the last two groups, but also those within the middle. And whereas I talked before about the formal transfer of electrons from one atom to another, now I'd just like to briefly introduce you to the idea of covalent bonds—that is, where you have electrons which are shared. So rather than formally transferred, they're shared in order for both species to obtain a full outer shell and the stability that comes with that. Let's take, for example, methane. This consists of two elements: one carbon and four hydrogen atoms. Let's consider those individual atoms in isolation. If we look at carbon (again, we're using the abbreviated form, so the helium nucleus followed by 2s2, 2p2) we can see that it needs, or requires, four additional electrons to complete its valence shell. If, on the other hand, we look at hydrogen (which, as you should recall, has the electron configuration 1s1), it needs to gain one additional valence electron in order to complete its outer shell. So if carbon shares all four of its valence electrons with four hydrogen atoms, such as this shown, this enables the completion of the outer shell for both atoms, or both sets of atoms. The hydrogen gains the electron it requires to create 1s2, and the carbon can share the four electrons from the hydrogen to create the 2s2, 2p6 outer shell. So hopefully, you can appreciate in this scenario, where the actual loss of individual electrons is not energetically preferable, sharing, or covalent bond formation (which is what I'm showing you here), is the preferred way in which atoms can achieve the energetic stability associated with a full outer shell. The bonds between the carbon and the hydrogen are covalent bonds. And I'll just want to briefly draw your attention to the shape of the molecule, as I've shown here. If you recall, if we look back at the anionic and ionic systems, I talked about the idea that they form these crystal lattices, or structures, where you have multiple ions all able to interact with each other electrostatically. However, in the case of covalent bonds—and this is very important for macroscopic properties—the bonds are directional. The carbon is bound to the hydrogen and nothing else. And so therefore, you don't have the same macroscopic interactions, which means that the methane exists as a gas rather than as a crystal structure.

03:22 So due to their electron requirements, hydrogen will always form one covalent bond and is said, formally, to have a valency of 1. Carbon will always form four covalent bonds and therefore has a valency of 4. And as we will see when we actually start looking at things like hybridization, it'll make you realize the importance of the carbon in organic chemistry. Because whilst it's possible to form many thousand formula units, compounds, and molecules and so forth from other parts of the periodic table from different elements, carbon in itself is directly related to the millions and millions of different compounds which exist on this planet and is therefore one of the most—or the most—important compound because of its diversity.

04:22 So, question for you now: • Ammonia is a molecule that contains a single nitrogen covalently bound to hydrogen atoms. So what will the molecular formula for ammonia be? • What, therefore, is the valency of nitrogen? • And a follow-up: Hydrazine, which is a related molecule, contains two nitrogen atoms and hydrogen atoms. The nitrogen atoms are joined by a covalent bond. What you should do as an exercise is draw the bonding and give the molecular formula for hydrazine.