welcome to the first video for chapter 4 section 2 covalent bonding uh in this video i'll be talking about some covalent molecules including a very cool one buckminster fullerene or buckyballs which is just 60 carbon atoms covalently bonded together into a sphere that you may recognize from soccer balls and this this molecule buckminster fullerene or buckyballs is uh named after buckminster fuller who is an architect who designed the uh geodesic spheres that are used um for a whole bunch of things and uh gave gave this molecule the name all right so the learning objective for this video is to describe the formation of covalent bonds so first off we'll define a covalent bond and a covalent bond is a bond that forms due to the attraction of atoms that results in the sharing of a pair of electrons between two atoms this is different than an ionic bond that we talked about previously where that's an electrostatic attraction between two charged particles in a covalent bond there are no charged particles um the the electrons are being shared they are not uh they have not been transferred from one atom to a different one before we talk about how covalent bonds form i want to talk about some characteristics of covalent compounds the first thing to note is that they have much lower melting points in boiling points than ionic compounds they are often liquids or gases at room temperature where ionic compounds are usually solid a sort of typical covalent compound that you can think of if you are curious or or need to kind of think of something concrete is water water is a comp equivalent compound um the fact that these have lower melting points and boiling points than our typical ionic compounds tends to indicate that they have weaker bonds than ionic compounds they are weaker than ionic bonds also many covalent compounds are actually insoluble in water that's not always the case water is does not fit that but many uh covalent compounds do fit this characteristic where they're insoluble in water they are also poor conductors of electricity in any state so what this means is that there's no ions or charged particles flowing around there's no ion movement um and in any state that means in solid liquid gas or dissolved in water there's still no ion movement and that's that's different from what we saw with ionic compounds what this tells us that is that the smallest chunk of a covalent compound is not an ion it's in fact a molecule and it's still neutrally charged um generally we're going to think about covalent compounds our covalent bonds are formed between two non-metal ions which means so our covalent compounds generally consist of two non-metal ions but what that really means is that generally covalent bonds form between atoms or elements with similar similar electron affinities or ionization energies and that just tends to happen in the non-metal part of the periodic table what this means is that there is not one atom that is significantly more likely to give up or gain an electron than the other one so they are much more likely to share the electrons a little bit more equally all right so let's talk about how covalent bonds tend to form this is a diagram that can be super helpful to explain a whole lot of stuff in chemistry it's a really really helpful diagram uh it basically depicts the the relationship between the distance between two nuclei and here we're looking at hydrogen atoms versus the energy on the x-axis is what we call the internuclear distance so that's essentially just right that's the distance between these two nuclei and it's given here in picometers and then on the y-axis we've got the energy and that's given in in joules um we did have to choose scientists had to choose a a zero point a reference point and so what we had to do was pick basically the spot where um we're gonna say that these these nuclei are so far apart they're essentially infinitely far apart that is the point where the attractive force between them is zero and the energy of the system is uh zero um and the reason we did that is because we had to pick a reference state that's the same for everything and where there is no interaction is the only point that it's the same for everything uh so that just means that all of our energies are going to be the ones that we're interested here are going to be negative so it's a little bit unfortunate but it's just how that works all right so what this diagram shows is how as these atoms get closer and closer together the energy of the system changes so as we see when the atoms approach each other and they begin to interact the energy decreases so what's going on here is that remember that electron density or the the volume of space around the atom around the protons around the nucleus that we tend to think that electrons are occupying is actually a distribu a distribution of probabilities so what we think is they're probably in this volume most of the time but sometimes they're not as these volumes get closer and closer together there's a greater and greater chance that in fact the electron from this nucleus or this sorry this atom is going to be spending time around this atom and it'll be interacting with that nucleus that's an additional attractive force that that electron can experience and that lowers the electron so the um the energy very slowly goes down and then we get to a point where essentially the s orbitals overlap so when the s orbitals begin to um when the s orbitals begin to overlap what we see is that now there's actually a much greater probability that either one of these electrons could be spending time around the other nucleus at any given moment and that's going to start to really drop our energy down the more overlap we have the more likely it is that that these electrons are experiencing these additional attractive forces so they're they're experiencing attractive forces between their own nucleus and now the additional one in other words they're beginning to be shared and those additional attractive forces decrease the energy of the system and so eventually we get down here to where we see this minimum what happens at this minimum is that if we tried to force those uh atoms any closer together or they just randomly moved closer together the positive charges in the nucleus would start to repel each other and the energy would begin to increase rapidly this increase in energy so remember that that the vast majority of space in an atom is empty right the the nucleus is incredibly tiny compared to the volume that we think of of the atom but this repulsive force between the nuclei is essentially why you can't stick your finger through tables um it's this repulsion between the nuclei in your finger and the nuclei in your in your table so that's what's going on this minimum here if we come back to this minimum down at the bottom of our what we call an energy well is uh is the is represents both the energy that's released by the formation of this bond and it will uh it represents the distance between the nuclei that is the um the most stable nuclei uh the the most stable internuclear distance um for this bond or in other words the bond length um between two hydrogen atoms here so it's important to keep track of the changes in energy that accompany physical changes or chemical changes as we see here that's one of the key things that we look for in chemistry one of the key things that we do to sort of help us understand the changes in the world around us so this can be a little bit confusing but essentially as we see with the formation of this bond and this energy well when we form a bond or when atoms form a bond that releases energy to the system or sorry to the surroundings from the system to the surrounding this is what we call an exothermic process um and exo is uh i think it's i think it's a latin it might be greek but it's a latin prefix that just means exterior right outside so exothermic just means releasing energy uh thermic to the outside if you break a bond that actually requires energy um from the surroundings and that is what we call an endothermic process endothermic so you can think about this with um as we see when the when the atoms move close together and then they form a bond um the energy decreases and and that essentially tells us that the energy is being released to the surroundings if we wanted to then break that bond and return these atoms to their unbonded state we would have to put in enough energy to uh to get the atoms out of this energy well