WATER |
Oops. Sorry.
Where was I? Oh, yes.
Because chlorine is more electronegative than hydrogen.
|
![]() |
Right! Water is a polar molecule. You will recall that hydrogen and oxygen normally make a covalent bond when they come together. But the oxygen attracts the electrons more strongly than the hydrogen.......because oxygen is more electronegative than hydrogen (hydrogen is such a whimp).Yes. The oxygen pulls electron pairs in the covalent bond closer to itself than to the hydrogens.I see. And that means the oxygen will be slightly negative because it hogs the electrons, even though they are shared.That's right. The hydrogens, both of them, will have a small positive charge because their electron is not covering the proton very well. This leaves the positive part of the hydrogen (the proton) slightly exposed. This slightly exposed hydrogen atom, with its slightly exposed positive charge, can exert an electrostatic force on other atoms. |
![]() |
Yes. That is exactly what I mean. Electrostatics can work on any molecule with a charge, even a partial charge like these polar molecules. So other atoms, if they have any charge to them at all (even partial charges of their own) can interact. Got it?Sure do. Water is a polar molecule. It has a slightly negatively charged oxygen atom and two slightly positively charged hydrogen atoms. But only slightly. Neither the oxygen nor the hydrogens are ions. Right?That's right. They are like "almost ions". Sometimes this unfair sharing goes too far and the molecule ionizes. But let's not get into that now. Just remember that ALL water molecules have these partial charges that create polarized bonds and that's what makes water a polar molecule. |
![]() |
Yes. As a matter of fact, the solvation shield not only blocks
the ions from meeting. It also hides their charges. So, not only
are the two ions prevented from getting together, they don't even
"feel" their opposite ion hidden inside the solvation
sphere.
What about the other way around? What happens to the Cl-
ion?
|
![]() |
That's right.
|
![]() |
Well, we can't all be called Arthur or Merlin.
Very, very slight, is it?
|
![]() |
Yes. What happens to the water's energy?Oh, I see what you're getting at.The fire supplies energy to the liquid water and causes it to wiggle so much that the molecules go flying off. Yes, that's exactly right. The energy in steam molecules is enough to overcome any tendency for two neighboring molecules to stick to each other by the weak forces.I see. So steam has too much energy in it for water molecules to stick to each other by van der Waals forces, or even hydrogen bonds! |
![]() |
The colder it is the less energy there is and the less the water
molecules wiggle.
Liquid water molecules have about the same amount of wiggle energy as bonding energy, so they cohere to each other, but only briefly. That's why liquid water behaves as a liquid, taking the shape of the container. |
![]() |
Notice how these water molecules are arranged. The oxygen atom (the fatter circle in each molecule) is hydrogen bonded to the hydrogen of another water molecule. And so on and so on. That causes the water molecules to set up into pretty patterns with six sides.I've never seen any six sided crystals in a chunk of ice.That's because neighboring ice molecules push against each other and break up the crystals before they grow big enough to see. To make big six sided crystals of ice you have to freeze the water very gently and give it plenty of room to grow so it wouldn't bump into neighbors. |
![]() |
Right! That's all it could be. Here's what the molecule would look like. I've included the electron cloud from both atoms. See how they overlap? Together the two atoms make a line. H2 is so simple that a line is the only shape it could possibly be. |
![]() |
OK. Carbon is at the center of the Lewis structure and has the
4 hydrogen's surrounding it.
Right. You're familiar enough now with Lewis structures to know how to arrange those shared electrons. There's nothing new to that.Yeah, but the way I have it drawn, it looks flat. Is that right? Is methane flat?No, it is not. Real molecules have all three dimensions to work with. So do you. Now we get to use VSEPR theory to predict the correct shape. The whole point of VSEPR is to arrange the electron pairs to be as far away as possible from each other. |
![]() |
Right. Because we are dealing in all three dimensions, the angle between these bonds is actually 109 degrees, not 90.Oh, I think I'm beginning to see what you mean. It just isn't easy.I agree. The problem is that there is no easy way to draw a three dimensional object on a flat surface. Notice how I've drawn these bonds. I picture the hydrogen directly above the carbon as being in the plane of the paper (blackboard or screen). Notice that the line connecting the carbon to that hydrogen above it is just a simple line. It is the same width from top to bottom. |
![]() |
I think I'm getting the hang of it. Take that hydrogen below (and
slightly to the left) of the carbon. Its wedge is backward
from the two on the sides. It is as if that bond were sticking
out of the page. Is that right?
Absolutely! That's perspective. Now you can imagine (by looking at this picture) that the hydrogen I've drawn below the carbon is sticking out of the page....and the two on the sides are sticking into the page, behind the carbon. But the hydrogen above the carbon is in the same plane as the carbon atom. It isn't sticking in or out of the page so its bond is the same width - not a wedge.Right. |
![]() |
Was it?
No, of course not. But it is pretty. Now let's get back to methane.Methane is shaped like a tetrahedron. The carbon is in the middle and it arranges its orbitals to be as far apart as possible. So they form a tetrahedral pattern. The hydrogens are stuck on the ends of carbon's four orbitals, so they are arranged around the carbon in a tetrahedron. Easy. |
![]() |
OK. First I draw the Lewis structure and assign pairs of electrons to form the bonds. Gee, it looks a lot more complex than methane. There are double bonds. And now I have to deal with oxygen atoms. They have L-shells as the valence shell. That makes them more complex than hydrogens. Help!! |
![]() |
Right. Now back to our problem. Double bonds in CO2, or any where, make a
SINGLE repulsion axis. By that I mean they work together to produce
a single (but pretty powerful) axis of repulsion.
|
![]() |
Give it a try.OK. The Lewis structure looks like this.I'll give the oxygen the o's and hydrogens the x's (for the electrons). |
![]() |
Well, all four of them will want to be as far apart as possible.
They will form a tetrahedral pattern. Just like methane. That
doesn't seem right.
But it is right! (Or very close.) Even though the oxygen only has two atoms attached to it, it has four repulsion axes. The lone pairs still make orbitals and VSEPR shapes them all into a tetrahedron.But the lone pairs don't have atoms attached to their end.Right. But they ARE Electron Pairs in the Valence Shell which Repulse each other. So you have to count them in the VSEPR. However, you are right to point out that there are only two atoms. Those two atoms, the hydrogens, are held by the two bonding orbitals. |
![]() |
What about hydrogen bonds? Do they affect a molecule's shape?
They can. They affect the shape of ice. Hydrogen bonds are very "linear". Look at the way a hydrogen bond is made. You will see that it MUST make a straight line between the electronegative atom (which draws electrons towards it), the hydrogen and the other atom that is connected by the hydrogen bond. That affects how the molecule bonds to another molecule. Hydrogen bonds must line up. |
![]() |
I see what you mean. Hydrogen bonds are lines. OK, what about the bonds made by van der Waals forces?