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.