# Structure of water, Why?

sonhouse
Posers and Puzzles 28 Apr '07 11:54
1. sonhouse
Fast and Curious
28 Apr '07 11:54
So you have all seen the mickey mouse image of the water molecule, nice fat circle for the Oxygen, two smaller ones at the mickey mouse points. All well and good. It's that magic 105 degree angle between the two H's that has me puzzled. For instance for a quick totally differant for sure analogy, you have a pith ball with a positive charge on the end of a glass rod and two smaller ones with the same charge with threads to hold them near the bigger one. Now like charges repel of course so if you have the two little ones on threads hooked to another rod and you try getting them close to the larger pith ball, the small ones flop around at any odd angle and there is no preferred direction of motion or resting point, of course there is no spinning moment with static charges either, but the point is there is no preferred spot for them to come to rest except to be as far apart from one another as the system will allow. Not so with the H's.
You get two H's and one O together at the right temperature and BOING, they pop together right at the 105 degree point every time.
So what is it exactly about the H's that they decide to be at this apparently minimum energy point that happens to give water its vital functions in biology. With the two off center like they are, the molecule aquires a differance of charge on one side vs the other. It would only be neutral if the H's ended up 180 degrees apart, that would make it as close to zero net charge as you could get but with them at 105, the whole thing is off center and you get pluses on one side and minuses on the other so the pure water molecule can rip things apart which is why DI (De-ionized water, like totally nothing but H20's and no impurities) will corrode stainless steel fittings and you have to use stuff like teflon instead. But what is it about the H and the O's that mixed together ends up at that 105 degree angle?
2. 28 Apr '07 12:561 edit
Originally posted by sonhouse
So you have all seen the mickey mouse image of the water molecule, nice fat circle for the Oxygen, two smaller ones at the mickey mouse points. All well and good. It's that magic 105 degree angle between the two H's that has me puzzled. For instance for a quick totally differant for sure analogy, you have a pith ball with a positive charge on the end of a g what is it about the H and the O's that mixed together ends up at that 105 degree angle?
A single oxygen atom has 6 electrons in its outer electron shell.

When bonded to 2 hydrogen atoms the oxygen shares another 2 electrons (1 from each of the hydrogen atoms) so in a water molecule we consider the oxygen atom to have 8 electrons in its outer shell.

These 8 electrons exist in pairs for reasons which i'm a bit foggy on now but knowing it isn't crucial to understanding the molecular structure of water.

So we have an oxygen atom with 4 pairs of outer shell electrons orbiting it (2 of those pairs of electrons also happen to be orbiting a hydrogen atom nearby).

Now each of those pairs of electrons repels each other so in 3-dimensional space they arrange themselves in such a way that they are as far apart as possible. This arrangement is called a tetrahedral arrangement and consists of 3 pairs of electrons pointing down in a tripod like manner and the 4th pair pointing straight up. The angle between any pair of electrons is around 105 degrees.

Try to model this with plasticine or fruit and toothpicks.

Ok now 2 of our electron pairs happen to have a hydrogen nucleus attached to the end of them. In my model i have an apple with 4 toothpicks sticking out of it and 2 of the toothpicks have a grape on the end. The non-bonding electrons (without the grapes) are effectively invisible so we are left with a "mickey mouse" like shape to our model.

In summary: There are non bonding pairs of electrons orbiting the oxygen which behave almost exactly like bonding pairs. It's not the 2 hydrogen atoms that are repelling each other, the pairs of electrons in the H-O bonds are repelling all other pairs of electrons. As there are 4 pairs of electrons in the outer shell of the oxygen atom they arrange themselves in a tetrahedral arrangement where the angle between any pair of electrons is about 105 degrees. Hope this helps.

There are undoubtedly better resources on the net to explain this, even a highschool chemistry textbook should do a better job than me.
3. 28 Apr '07 13:26
I'll vouch for that analysis -- it's what I learned in chem too. The only thing you missed is that the proper angle in a tetrahedron is actually more like 109.5 degrees. Those unbonded electron pairs have slightly greater repulsion, so they push the bonded ones a little closer together. In a symmetric tetrahedral molecule like methane (one C, four H), the angle will be right on.
4. DeepThought
28 Apr '07 13:33
Originally posted by Marsan
These 8 electrons exist in pairs for reasons which i'm a bit foggy on now but knowing it isn't crucial to understanding the molecular structure of water.
Electrons are spin half particles. This means that there are two spin states available to them. They are fermions, which follows from the spin statistics theorem, which means that you can't get two of them into the same quantum state. So each atomic orbital can fit two electrons into it, one with each spin alignment.
5. sonhouse
Fast and Curious
28 Apr '07 17:29
Originally posted by CZeke
I'll vouch for that analysis -- it's what I learned in chem too. The only thing you missed is that the proper angle in a tetrahedron is actually more like 109.5 degrees. Those unbonded electron pairs have slightly greater repulsion, so they push the bonded ones a little closer together. In a symmetric tetrahedral molecule like methane (one C, four H), the angle will be right on.
I thought the electrons were in a state that you could not determine precisely the location and therefore you would refer to the electron and its orbit as an electron cloud. So if its not known where it is, why are the two H's fixed in place? Why aren't they going around following the electrons? What keeps the two H's from sliding around and therefore changing the orientation? I sort of see how they are 105 degrees apart but aren't the three molecules fixed in place compared to each othere? As opposed to the H's continually being 105 apart and the two part system sliding around the Oxygen.
6. AThousandYoung