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0:02
Hi this Professor Scherr. This is going to be a little lecture on chemical bonds. I'm assuming that you have read in the book and done your Mastering Biology for the sections up to the bonds at least, and so then I will be just using terminology you should already be familiar with. So when we look at chemical bonds the most important thing to remember about atoms in general, really, is that their behavior is going to be dictated by their outer shell. Their outer shell's also called the valence.
0:37
And the electrons out there are called the valence electrons. And so the shells are basically going to be moving to be full. We talk about this on your Mastering Biology. So you can see in different shells there's different numbers that need to be met. So the first shell we're looking at two electrons max. And you can see both these atoms here have two electrons so they're full the first shell, second shell going to be 8. You can see we only have four in the carbon. But we have in the 8 neon.
1:07
So carbon needs more to fill its shell. Neon is set. And for our purposes the third shell we're going to be looking for 8 in that as well. How do we fill these shells? There is really two main ways that these shells can be fulled. We can either share electrons with other atoms. So some of time the electrons around one atom and some of the time electrons around the other atom. And that will count. Or you can completely transfer electrons from one atom to another. And all these are going to
1:41
result, the sharing or transfer, both result in chemical bonds. Which is how you make atoms stay close together. So if we look on the periodic table you'll notice that with the exception this column on the far right, everything else is missing at least one electron. The far right are called noble gases. They do not react, they are not going to form bonds because they are already full. But all the other atoms they are going to either lose, gain, or share electrons to make their outer shell full.
2:11
When we talk about biology, biological systems, there are 4 atoms that are most important - four elements. So those are going to be carbon, nitrogen, oxygen, and hydrogen. So I remember it as CHON; carbon hydrogen oxygen nitrogen. All these guys are going to covalently bond. Okay. So,
2:44
Co, together, and have we have valent which is your valence electrons. So here we're going to have two valences being together, so they are sharing elections. But if we look at an example of that, let's take our simplest atom, hydrogen. I'm not drawing the orbitals I'm just going to draw the electrons. So here's a hydrogen atom, it needs another electron, its missing one there. So let's just put it and paired up with another hydrogen atom. Which also needs an electron, and so they can bind
3:18
and make a covalent bond. So now the electrons are shared around hydrogens.
3:28
So if you count, hydrogen is going to have one electron, two electrons and same for the other hydrogen. We can actually get rid of the empty spots because now they are sharing electrons and both of them have two electrons, at least some of the time. We would write that using a couple different formulas, or methods. So one way we could write this would be to write what we call the structural formula. That's going to be a solid line for a covalent bond. We can also write a molecular formula
4:08
for this. So this tells us there's two hydrogens in this molecule. So you need to pay attention to what I'm asking you to draw or write. So that's a pretty simple. This is a single covalent bond that's going to be in the hydrogen. Let's look at another molecule. So we'll look at oxygen. So oxygen has an atomic number of 8. So that means two in the first shell which gives us 6 in the outer shell. If you don't know what I'm talking about go back to your Mastering Biology and look at that.
4:50
So here's our oxygen, it's missing two electrons. Let's take a hydrogen, in red, hydrogen has one electron here's another hydrogen with one electron. So what can we do? We could share electrons between the oxygens and the hydrogens. Now if go around and count for oxygen, we have one, two three four five six seven 8 electrons. So that's full. Hydrogen has one, two. Hydrogen has one, two. So now we have two full valence shells, so that is a stable molecule. And we could draw at like this, that's
5:29
the structural formula and the molecular formula as you're probably familiar with is H2O, which indeed is water. So these all have been single covalent bonds. Now let's look at what happens when you share electrons multiple times at the same atom. So here's our oxygen again. Let's put another oxygen up there. So both of them need two electrons so they could share twice. Now we have full shells 1, 2, 3,4, 5 ,6, 7 8. 1 2 3 4 5 6 7 8.
6:03
And so this would be a double covalent bond, we just draw it with two solid lines. And this is 02 that's the oxygen gas in the atmosphere. So, so far we've looked at hydrogen oxygen. Let's look at the other two elements that are part of our CHON. So we will look at nitrogen next. So nitrogen has an atomic number of 7. Which means that has 2 in the first which leaves us with 5 in the second shell which means it has three bonds it needs to make. So let's do the simplest thing.
6:39
We'll just take some hydrogens in. Hydrogens again needs one. If we bring three of them in, they can share, share, share. And now we have eight electrons for the nitrogen in the outside and two electrons for each hydrogen, because it's only one shell. This is how you draw this. NH3 which is the gas ammonia. And then finally our carbon, probably the most important element of life. Carbon has an atomic number six, which means that has four on the outside two
7:15
on the first shell. Which means it needs four bonds. So this says very versatile. We can bring some hydrogens in. You don't have to draw the electrons. This is just so show you where they are sharing. So this is methane gas, CH4, like natural gas that runs your stove. We can also draw something with a carbon to oxygen. Now oxygen remember needs two, carbon needs four. So we've already got them sharing one. What if we share again.
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So now the oxygen has 8 total, it has its two bonds, and carbon has two left, so if we bring in a hydrogen and a hydrogen we can make a molecule that looks like this. In this case this is formaldehyde.
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H2CO. You don't need to know any of these molecular formulas particularly, I'm just giving you some examples. Then you can start to build very complex molecules, and this is a pretty simple one, but you can see we're gonna have all the CHON in here, some double bonds, some single bonds. You can also have triple bonds with nitrogen. If you work it out you can triple bond nitrogen for the nitrogen gas. But you see here, so basically your keys are to understand that every carbon has
8:50
four bonds it needs to make, every hydrogen has one, every oxygen has to needs two and every nitrogen needs three. So if you can remember this you'll know how many you need to look for. So now you know nitrogen needs three' so 1, 2 ,3. This nitrogen is fine. Hydrogen needs one. This hydrogen here has one so it's fine. This hydrogen has one, its fine. Carbon needs four - 1, 2, 3, 4. That carbon is good. This carbon needs four - 1, 2,
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3, 4. So that carbon is good. This
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we just haven't written the bond in between it. But oxygen needs two, so 1, two, so it's good. Hydrogen just needs one, so it's good too. You can see go to the whole molecule to see that. And just to remind you we're not actually talking about these rings we're talking about orbitals. That's what's being shared, is the election clouds are being shared between the two. When we talk about covalent bonds we have to talk about the property electronegativity.
9:55
Which is basically how strongly atoms hold onto their electrons. So high electronegativity means they are electron hogs, low electronegativity means they give up electrons pretty easily. So this affects how electrons a distributed between the two atoms that are sharing them. So here is an electronegativity table. And you can see the trend generally for the table is up and to the right, the higher electronegativities. So I to think of electronegativity as they love electrons.
10:26
They are going to keep them them. What you do, you don't need know these numbers, but you can take the electronegativities and you can compare them to see what's gonna happen with the electrons in the bond. So in a situation where we have atoms with similar electronegativities, such as carbon and hydrogen, they are going to share electrons relatively equally. This is going to be a really important combination that you remember. The covalent bond between carbon and hydrogen is what we call
10:57
non-polar. Non-polar bonds are ones that share electrons equally. Electrons around carbon as much time as they are around hydrogen. So if we had a molecule that had lots and lots of carbons and hydrogens attached, this is going to be a very non-polar molecule. Because every hydrogen carbon bonded sharing electrons equally, it's all distributed the same. On the other hand, when we look at an oxygen hydrogen bond, oxygen is much more electronegative than hydrogen.
11:43
Actually if we look at the numbers, oxygen is a 3.44 electronegativity while hydrogen is a 2.20. This difference is strong enough to where oxygen, loves the electrons, highly electronegative and hydrogen doesn't as much. So that mean the electrons are going to spend more time around the oxygen atom. This in turn means that the oxygen atom, in the bond, is going to have a partial negative charge, the electrons that are negative hang out there more often.
12:15
That means the hydrogen has a partial positive charge. It doesn't have the electrons on the hydrogen as often. This is a delta, the little funny looking thing here, it's a Greek lower case delta and that's how you draw a partial. So it's not a complete charge, it's only a partial charge. So this is called a polar covalent bond. An oxygen hydrogen is really important one, that's polar covalent. We also have nitrogen hydrogen bonds, so that will come into play sometimes.
12:48
The nitrogen is stronger than hydrogen. Then we can also sometimes have carbon oxygen bonds. It's not quite as much difference but still the oxygens stronger than the carbon. You can draw it either way. But you'll notice every time the more electronegative atom is the one that has the partial negative charge. So if you look at it visually I like to see how the elections are distributed. Sometimes we use colors. So here's an example of that methane,
13:21
CH4. All these are carbon hydrogen bonds. You can see green means equally distributed electrons. So this is a nonpolar covalent bond. On the other hand, this is I rainbow you can see, or sometimes it's just white, blue, and red. This is an oxygen and hydrogen, this is water. You can see oxygen and hydrogen are quite different. This means that the electrons are going to hang out more around the oxygen than they are the hydrogens, so you can see the difference in color
13:54
here. So it's polar, it has a partially negative pole and a partially positive pole. So what is this electronegativity matter? In a lot of ways it matters because of water. Water, H2O, has the partial charges as well. So it's an oxygen hydrogen. So we said that means that the oxygen is partially negative and hydrogens are partially positive. Let's draw another water molecule and put in the partial charges. We'll look and see what happens between these two. If you remember anything
14:33
about charges, you know that opposites attract. So our partially negative is going to be attracted to our partially positive, in two different molecules. So the partially negative oxygen, partially positive hydrogen. And this can happen over and over again until you get this lattice work of bonds with the water. This is what makes water unusual. So this is our hydrogen bond. Hydrogen bond because hydrogen is involved in the bond and its partial charges.
15:05
There is no electron sharing, it's just an attraction. Whereas this, recall is our polar covalent bond. So there's both bond at work in this situation. On its own hydrogen bonds are very weak, but when you put a bunch of them together they have a strong impact and you'll see that with the water unit on Mastering Biology and D2L. Sometimes the hydrogen is not involved but there are still partial negative and partial positive charges attracting on two different
15:39
parts of a molecule or two different molecules. And we can call that dipole-dipole interaction. Dipole meaning it has a positive side and negative side. and then it ??? between two of those. So when hydrogen's involved it's specifically called a hydrogen bond. Here's a prettier picture water, you can see the partial positives, the partial negative. And then if you drew this we'd have this partially positive hydrogen attracted to this partially negative
16:13
oxygen. Here's what it looks like in three dimensions with the colors. So you see the blue and the red, opposites attracting. There's no electrons shared, just attractions between it. You can make millions and millions of water molecules all do this, all together, and breaking and forming and breaking and forming, which gives what it's unusual properties. Hydrogen bonds don't only happen in water, so here's an example a hydrogen bond between ammonia and water. Both of them are polar
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molecules. The nitrogen is partially negative, this hydrogen is partially positive and hydrogen bond is represented by a dotted or dashed line, that's how you're going to draw it between the two. Whereas the solid lines, those are going to be your covalent bonds. The next type of bond is an ionic bond. You've already learned about ions on Master Biology. There's four main steps to forming an ionic bond. First step is that electrons are transferred
17:17
between atoms. So one loses electrons one gains electrons. After the transfer they are now ions. Then, each ion is attracted to the ion of the opposite charge, so that's step 3. And finally the ionic bond forms. So it's electron transfer, ion nformation, attraction, equals ion bond. Classic ironic molecule are the salts, including table salt, which is sodium chloride. So let's see how that that formed. Sodium has an atomic number 11 which means it has one extra
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on its third shell, one extra electron. Chlorine has an atomic number of seventeen. So it has 7 on its outer shell, it's only missing one. They don't share because chlorine is very, very electronegative, so it strips the electron from sodium. So sodium actually loses its electron completely to chlorine, sodium, minus 1 negative, so that means is positive. Chlorine has gained one negative so that means it's now negatively charged. These are two ions, they're going to attract,
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we don't really have a way of representing that by dotted lines or dashed lines, but then you can draw it NaCl. That's the compound that's formed. ??? molecule because there's no covalent bonding but it is a compound. And that forms your table salt. Here's a prettier version of that. You can see the crystal forming from the salt, you'll also need to know the word cation and anion. There's more information about this on Mastering Biology as well. The way I remember anion is a
19:03
negative ion. And the cation, the 't' in cation is like a plus, which means it's becoming positive. If we look at a problem like this, you should be able to predict what type of bonds occur between different elements. Take a look at it, see if you can figure it out. When you're looking at ionic basically looking at who needs to just lose a few or lose one and gain one. That's the classic example. If you're looking at gaining three or four, it's not going to happen.
19:40
Two might happen, but its more unusual. So really you're looking for losing one or gaining one. So in this case you can see lithium here needs to lose that electron and fluorine here needs to gain the electron. In fact thy're in the same columns as sodium and chlorine. This means lithium it's going to become positive ion. And fluorine will become a negative ion and then you should get lithium fluoride sure salt, when it forms an ionic bond. Covalent bonds are the strongest
20:18
bonds but hydrogen and ionic bonds are also very important. When you have a lot of them they make a big difference and they really helped the shapes of the molecules form. So we're going to see that over and over again. The shape of the molecule is so important for its function. How the shapes are held together is primarily through hydrogen bonds and ionic bonds. Additionally there are attractions that are weak called Van der Waals interactions.
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But also important when there's a lot of them. They form because molecules have electrons moving all around them all the time, atom to atom. At any given time electrons may be concentrated in this spot, but not in that spot. Giving them negative and positive charges, very briefly though. Cause then the electrons leave and then it becomes neutrally charged again. And so what this can do is help hold parts up the molecule together in itself.
21:15
Additionally it could help hold this pink molecule to the blue molecule. SO it has some fleeting charges itself. And the Van der Waals interaction briefly holds them together until the charge goes away. These are important when there's a lot of them. For example, a gecko, it doesn't actually have any sticky on its toepads. The way that those pads help it the climb up a wall is because they have in little hairs on the bottom them,
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creating lots and lots surface area for Van der Waals interactions between the surface they're climbing on and their toes. If you recall, structure and function is one of our big themes that they're related, and you can see that very clearly in molecular shapes. So whether it's a small molecule like water or methane here, or very large molecule like a protein, the shape of that molecule is dictated by which atoms are there, how they're arranged, what types of bonds are made,
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and that dictates what kind of properties that molecule's going to have. So water behaves very differently than natural gas does, which behaves very differently than a protein. Here's an example, if we have a molecule here that needs to bind another molecule in order to work on it, do a reaction let's say. And there are square molecules in the cell and triangular molecules in the cell and oblong molecules in the cell, you could see which one is most likely to fit and work with that
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black protein. The one that had the right shape. So most likely the oblong molecule be the one that combined. Particularly if the charges, because of what atoms are there, match the charges of the opposite atoms. If you have a positive in this side you have a negative on that side. You have a negative on this side, you have a positive here and that will help attracted it as well. So really shape is responsible for the function of the molecules.
23:06
Molecules undergo chemical reactions with one another. And really it's the making and breaking of covalent bonds that counts as a chemical reaction. So dissolving salt in water, which is breaking of ionic bonds, is not chemical reaction. You can see here though here are some covlant bonds. And these are new covalent bonds. So they're broken in the reactants and then they're reformed the products to make new molecules, new product molecules. So you notice also that there's four hydrogens to
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begin with and for the end. Two oxygens and two at the end. So we don't lose any matter, we're just rearranging it. Reactions can go in both ways. So let's say we have A plus B goes to C. And C can go back down and be broken down into A & B. When the rate of the forward reaction, the combining A & B, is equal to the rate a the breaking down of C, that is what we call chemical equilibrium. And remember equilibrium does not mean static, it does not mean staying the same, not moving.
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These reactions are going to continue going back and forth and back and forth. It'll just be the rate this way is the same as the rate that way. So then they stay in proportion to each other. It doesn't necessarily mean that all the concentrations are same either. It's just whatever rate you're building the C you're breaking down the C, and that's chemical equilibrium.
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