all right we now begin the next phasee of our chemical existence oh wait a second we're not really changing phases we're still going to remain solids or at least Solutions throughout most of this lecture and by that I mean us we physically being solid walking talking big bags of solution our subject however is going to change from whoa did I just like totally Le everybody there I think I totally lost everybody there because I just lost myself I don't know about maybe I should erase that but I don't know anyway um comparing ionic molecular compounds if we're going to compare ionic and molecular compounds that means we're ready to talk about molecular compounds molecular compounds are compounds that consist of molecules molecules are discrete units of a compounds held together by calent bonds the chemical formula represent something that actually exists this is a big change from our ionic compounds remember our ionic compounds existed in crystals and when we wrote their formula we are writing a ratio when we write a formula for a molecular compound that exists as a molecule such as water this formula represents how the compound physically exists water is actually one atom of oxygen coent bonded to two atoms of hydrogen when we drink a glass of water we're drinking trillions and trillions of molecules that consist of one oxygen atom attached to two hydrogen atoms so this is a big change in how we think of things and how our compounds exist not to mention the force that holds them together calent bonds versus ionic bonds there are other differences between calent and ionic compounds as well for example the melting point for an ionic compound is a general rule is a general rule is much higher than the melting point for a molecular compound the same is true as your boiling point the type of bond that holds an ionic compound together is an ionic bond as opposed to a calent bond for a molecular compound ionic compounds at room temperature are nearly always solid nearly always solid for a molecular compound how they exist at room temperature is going to vary some are going to be solid some are going to be liquids and others are going to be gases much more variety than ionic compounds which are going to be solid at room temperature and those are your fundamental differences we won't be labor this point let's move on to talk about calent Bond formation and the disassociation energy there are a couple different ways of thinking about a Cove valent Bond forming the simplest of these ways of thinking about it is to think about our formation of water you have oxygen and oxygen has six veence electrons so it needs two more veence electrons to complete his octet hydrogen has one veence electron and to complete its octet it only needs one more right because first energy level hydrogen only needs two veence electrons to have a full octet if that bothers you we'll get to it later right now just take my word for it hydrogen has a complete veent shell with two electrons so both hydrogen and oxygen can complete their octets and get what they want to out of life simply by sharing an electron with each other rather than one giving up its electron to the other they simply decide they're going to share they're going to form a joint custody situation and they form a Cove valent Bond where these two electrons belong to both hydrogen and oxygen that's the simple way of thinking it from it if you're somebody called a p chemist who's interested in the physics of chemistry and spends a lot of time doing energy calculations there's a couple more things you have to consider when you consider the formation of a calent bond first of all are electrons people do they really understand the are atoms people do they really understand the concept of sharing no of course they don't understand the concept of sharing blindingly obvious right so what really happens what really happens is this oxygen and this hydrogen gets so close together they bring their nucleuses so close together that the probability of the electron on the hydrogen belonging to the hydrogen nucleus is equal to the probability of that electron belonging to the Oxygen's nucleus the Oxygen's electron is brought so close to the hydrogen that the probability of that electron belonging to the oxygen is equal to the probability of it belonging to the hydrogen in other words it's like some really bad trashy romantic novel everybody read a bad trashy romantic novel no you're really wondering what I read in my free time now aren't you but anyway if you've read a bad trashy romantic novel there's usually a line in there saying their bodies were so intertwined they can't figure out whose limbs belong to whom right that's what goes on when two atoms form a calent bond their nuclei are brought so close together that they can't determine which of those two electrons it belongs to mathematically we would simply say that the probability of that electron belonging to either nucleus is equal that's the P chemist way of looking at the formation of a coent bond me I'm all hell prda they Shar bearing their electrons for the common good each working to his ability for the greater betterment of mankind down down with the aristocracy I'm sorry all right and none of you even know what I'm talking about because the Cold War ended in the 88 but at any rate when two Adams meet each other read a romantic trashy novel and decide to get together there are attractive and repulsive forces at place just like when two people meet meet there are an attraction between certain parts of them the nucleus of one atom is attracted to the electron cloud of the other the electron cloud of the other is attracted to the nucleus of the other however the two nuclei repulse each other they really don't like each other's noses the electron clouds are really attractive to each other my word have you seen are really repulsive to each other right the electron clouds both have negative charges so they're really repulsive to each other really what's up with the amount of hair on their hand anyway um but anyway right so there's some attraction wow beautiful gorgeous eyes what's up with that nose so there's a competing attractive force and repulsive force that play when two atoms want to form a molecule you can probably guess when that bond is going to form that bond is going to form when these repulsive forces are minimized and these attractive forces are maximized to be specific your bond distance between two nuclei is going to occur when the energy released is maximized at this distance this optional separation the attractive forces are maximized and the repulsive forces are minimized at this energy minimum this amount representing where the maximum energy released is where your Bond's going to form this picture is really crap actually I don't like it that much a key part of that bond is that whenever two atoms form a bond energy is released just like when two people hook up for the time for the first time let's say you they're two people who've liked each other in your group of friends for a long long time right and the tension between the two of them because they like each other is just like unbearable you you're just like to just stop talking to us about each other and get together and when they finally get together like there's a lot of energy released from the entire group yes same way for your atoms when two atoms form a bond they release a lot of energy however when en just like when energy is released when a bond is formed breaking UPS hard to do same way for Adams it takes a lot of effort to break up with somebody if you're polite otherwise I just recommend texting um or better yet just ignoring the phone calls eventually they stop o Dr Kelly's a jerk at anyway um it takes a lot of energy to break up with somebody same way for breaking a bond in order to break a bond you have to input energy the amount of energy you have to input to break a bond is referred to as the bonds disassociation energy to break the bond between two carbon atoms oops I should have done carbon carbon here between two carbon atoms you're looking at roughly 370 kles of energy to break that Bond conversely when the bond is formed three 370 K of energy is released carbon plus carbon forming a bond they're releasing 370 K of energy however when carbon and carbon break up all that energy was released ah they got to put it back in right we all know breaking up requires energy that's why we eat all those high carb foods Tada the amount of energy supplied for this breakup here is referred to as the bond disassociation energy look here I was going to all the trouble of generating my own graphics and I guess it pays attention pays for me to pay attention to the overheads I used before I begin lecturing um but this just goes to show that the bond disassociation energy is equal in magnitude to the amount of energy released when a bond is formed there's actually something I would note here when energy is released from a system it's negative so we going so going this direction it wouldn't just be forth 36 K it would be a negative 436 K because our system is losing energy to break a bond we have to input energy so that wouldn't just be 436 K it would be a positive 436 K going to come in real important in about four chapters this is just a table of different Bond disassociation energy so that you get a feel we won't do anything with chapter 8 all right now let's talk about polar bonds and electro negativity we've already covered electr negativity in a different chapter before we review electronegativity why don't you write down Pepsi write down Pepsi write down Pepsi P PSI I hope everybody is writing down Pepsi all right electr negativity is the ability of an atom in a molecule to attract shared electrons in a coent bond we've defined it as how badly an atom wants an extra electron how badly it wants to attract electrons we also said that it went from a scale of Zero no attraction for an electron to 4.0 irresistible we also said that electro negativity had two Trends both those related to effective nuclear charge the trends that electro negativity follows are um the same as they are for ionization energy and I hate the way your book does does this little Trend here on the Periodic Chart if you look at these electro negativity values they increase going that way and then they increase that way I don't know why they say decrease there increase going up right they have they have one Arrow showing an increase and one Arrow showing a decrease and that that just plays with your mind I think at least so it increases that way or it increases like that from cesium being your least electronegative to Florine being your most when in Conflict as a general rule left to right is going to take precedence in other words the left right direction is more important than up and down left right is more important than up and down so electro negativity increases that direction across your periodic table that means if we were to put this trend in context of electr negativity that means that the elements over here our Florine our chlorine our bromine our oxygen our n our nitrate our nitrogen our sulfur these elements are very attractive to electrons they want electrons a lot more than the elements over here we said previously that the one of the primary uses for electro negativity is to Define different types of bonds we said that an ionic bond occurs when the electro negativity difference is greater or equal to 1.9 classic definition of Electro of an ionic bond of course if that is that sodium chloride and if you look at the electro negativity values up on that Periodic Chart back there sodium has an electr negativity value of 0.9 chlorine has an electro negativity value of 3.0 so their difference is 3.0 - 0.9 or 2.1 2.1 is greater or equal to 1.9 so the bond between sodium and chloride is ionic if the bond is less than 1.9 it's Cove valent what we're going to do right now is we're going to break coal bonds into two different types we're going to say if the electro negativity difference is less than 0.9 but greater than or equal to 0.5 you have what is known as a polar calent bond if the electro negativity difference is less than 0.5 it's non polar calent so what do we mean by these terms polar calent and non-polar calent all right so let's take a look at a polar coal Bond the bond between hydrogen and chlorine hydrogen and chlorine form a calent bond hydrogen has an electr negative it value of 2.1 chlorine has an electr negativity value of 3.0 3.0 minus 2.1 giv us an electro negativity difference between the two of 0.9 that electro negativity difference 0.9 is greater than or equal to 0.5 and is less than no equal to and is less than 1.9 so that means that this bond between hydrogen and chloride is polar calent and you're saying great I did a lot of subtraction but why all right i' I've slep a label to it but why what does that mean good question scoob all right let's take a look here and clear out the the the garbage there I like to think of those electro negativity differences as being the age of our elements so we have a three-year-old chlorine playing with a 2 and a half well basically a 2-year-old hydrogen if the electrons in this Bond here are a ball that these two guys are playing with passing back and forth who's going to play with that ball most often if you've ever watched kids play you know that that three-year-old's going to be playing with the ball a lot matter of fact that 2-year-old it's never going to get the ball the only time that 2-year-old is ever going to see the ball is when Mom sticks her head out and says you let your brother play with that ball so then the three-year-old passes the electron to the 2-year-old and the 2-year-old gets to play with the electron until mom goes back inside the house once mom goes back inside the house what happens the 3-year-old takes the electron back the same thing happens with the electrons in these two chlorine is so much more attractive to electrons than hydrogen that chlorine plays with the electrons in this Bond a lot longer than the hydrogen does that means that part of the time chlorine has an extra electron it also means that part of the time Hy Ren is short an electron because chlorine has that extra electron part of the time it develops a partial negative charge and the hydrogen because it short an electron part of the time develops a partial positive charge we use the European baby face there that Sigma thing to represent a partial charge it doesn't represent a specific magnitude it just means that this charge is less than than one or or less or greater than NE has a magnitude less than one another way to think about this developing of partial negative and partial positive charges is I kind of think of it as a RAC trck model we have hydrogen we have chloride forming a bond and the electrons are going to move between them on a RAC trck well hydrogen has a really low electron negativity compared to chlorine so that means that the turnaround hydrogen is nice and sharp however the turnaround chlorine because it has such a large electr negativity is much wider right it's one of those Hot Wheels novelty tracks so our little race car of an electron is moving around this racetrack which atom along this racetrack is the electron going to spend the most time around it's going to spend the most of the time around chlorine it'll go around hydrogen sometimes but not nearly as much so that means that part of the time that chlorine has an extra electron and since it has an extra electron part of the time it has a partial negative charge that hydrogen because it has a part extra electron part of the time I mean because it's short an electron part of the time it develops a partial positive charge that's how I like to think of it your book likes to use these electron density graphs and they like to say look the electron density around chlorine is much larger than the electron density around hydrogen resulting in a partial negative charge and a partial positive charge if you have a partial negative charge on one end of your molecule or one end of your bond and a partial positive charge on the other end of your bond you have two poles we're used to the poles of the Earth right the opposite ends of the Earth and if you've played with magnets you know that the Earth's poles have opposite charges right or your magnets um poles have opposite charges so that's why we refer to this Bond where the electrons aren't shared equally as a polar bond we refer to this as a polar bond because the electrons aren't shared equally speaking of which your overhead has a mistake in it a polar coent bond has a unequal sharing of electrons that needs to be corrected there has an unequal sharing of electrons that means a pole exists your partial negative charge is always going to be on the more electronegative your partial positive charge is always going to be on the least electronegative we can represent the direction of the electron density or the directions the electrons in the bond want to stay in by um drawing an arrow this Arrow has a special shape this Arrow has a traditional Arrow Head but has a tail that looks like a plus sign this plus sign tail is always on the partial positive charge and the arrow always points to the partial negative charge this is going to be very important later on this drawing of the arrow is going to be our basis for determining whether or not our molecule is something called a dipole but when you have a polar calent bond you're going to draw an arrow from least electronegative to most electr negative oh be nice if my hand matched my words coming out of my mouth right most electr negative least Electro negative and you can always remember that because this end of The Arrow looks like a plus sign for a reason that's where your partial positive charge is that's where your partial negative charge is so that's a polar coent bond a polar calent bond is where the electrons are shared unequally between the atoms they're going to spend more time around the chlorine than they are the hydrogen again from a pure Schrodinger point of view we would say that the probability of finding the electrons in the bond is greater around chlorine than around hydrogen or the scatter plot of their densities or the scatter plot of their probable location provides more prob locations around the chlorine then around the hydrogen a non polar calent bond has an equal sharing of electrons whenever you have an atom bonded to itself the electro negativity difference between the two is obviously zero Which is less than a difference of 0.5 and what that means is that these two atoms share the electrons evenly they share them perfectly and because they share those electrons perfectly no partial charges develop and there's no polarity and no arrows to draw polar bonds are easy now I've made a big deal out of the subtraction in your book does let me give it to you easy let me make this easy for you easy peasy lemon squeezy here's how you can do it the quick dirty way of doing it you've got a metal bonded to a non-metal your Bond's going to be ionic you have carbon bonded to hydrogen or an element bonded to itself and the Bond's going to be nonpolar meaning the electrons are shared equally it's going to be non-polar calent anything else not in either of those two categories up above you're going to assume is polar calent if you remember this rules I should probably type that up because this is really the secret to what you're going to be doing for a large part not just of this semester not just of next semester but the two semesters to follow that in organic chemistry metal non-metal you're going to likely have an ionic bond carbon hydrogen or an element bonded to itself and you're going to have a non-polar Cove valent situation anything else you're going to have a polar coent bond wow after all that talking I could really use a seven up why don't you write down seven up seven Dash up seven up okay this happens to me in my face to-face classes as well I get so excited about a concept and know where I want to go on the spur of the moment that I outrun my coverage this um overhead uh really really just summarizes what I've said on a a previous overhead so great thing to study with again I included this in the handout to help you study you'll need this handout printed out when doing some of the homework here um one thing I would correct though if I were you in studying is I'd get that Arrow done correctly I like my Trends to match I would say increase right it just makes sense that if you're going to draw two arrows to show Trends Trend they're both going to be showing the same Trend increasing G whiz all right um that's it for this lecture that's all I wrote beep