water

[miriam_e]

Does anybody here know why the water molecule resembles Mickey Mouse? That is, why do the two hydrogen atoms stick onto the oxygen atom at 120° instead the apparently more obvious 180°. This has puzzled me since I was very young and I've never heard a good explanation of it. I've heard people say it is to do with the shapes of electron orbitals, but then usually dismissed as too complex to explain, which always sounds suspiciously like "I don't understand it myself so I won't reveal my ignorance to you". In my experience most concepts have fairly simple ideas at their heart. The trick is in finding the way to convey that.

Comments

[annie-lyne.livejournal.com]

I don't know much about the subject myself either, but it will have something to do with the way the electron shell on the oxygen atom is organized.

And it'll be the same reason the ozone molecule is shaped as it is too.

[(Anonymous)]

I know it has to do with overlapping bonding and anti-bonding molecular orbitals, and the electron energies of each shell and sub-shell - but darned if I can find an online resource (and I've been looking!) which explains it in plain English.

You can think of it as hybridized electron clouds and the distribution of energy around the molecule.

BEST RESOURCES FOUND:
http://www.lsbu.ac.uk/water/molecule.html
http://www.lsbu.ac.uk/water/h2oorb.html

Cheers, MFG

[miriam-e.livejournal.com]

Thanks Michael. It is going to take me a while to absorb those pages. They are a goldmine of info.

BSc. to the rescuuuuueeee!!!

[superchikka.livejournal.com]

Ok, so you know that in its unbonded state, Oxygen has 6 electrons in its outer orbital. Four of them are in pairs in sub-orbitals, and two are not. Two Hydrogens, forming each a covalent bond with Oxygen, supplies the two extra electrons to give a full outer octet of electrons. The two covalent bonds have two electons each. Now, you've got the three "balls" joined, BUT you also have, on the surface of the larger Oxygen ball, two pairs of elecctrons, which take up space. These must be "adjacent" to each other, because they are in lower-energy states and it would be lower-energy to keep them together than to be on opposite sides of oxygen, which is the configuration neccessary to have a linear arrangement of Hydrogen-Oxygen-Hydrogen. So we have electrons on part of Oxygen, taking up some space, and pushing away the Hydrogens. Each electron pair doesn't take up quite the space that an atom, would, however. Also at play is the two hydrogens pusing each other away. To get a balance between these three forces (grouping the electrons in this case as one force), they afre each 120 degrees away from each other... resuling in the mickey mouse appearance.

Re: BSc. to the rescuuuuueeee!!!

[miriam-e.livejournal.com]

Thank you. :)

That is a completely different explanation to what I was expecting. I'd somehow been led to feel (through my own ignorance or others' hints) that it was something to do with the shapes of the electron shells. But your explanation has a nice simplicity to it that I can "touch". I can just push and pull those charges in my mind as they wobble and spin about and I can feel that could well tend to balance at that angle. Cool!

Of course I now have more lines of questioning (why electrons pair -- another thing I often wondered, why keeping the two outer-shell electrons together maintains lower energy than letting them go elsewhere, etc), but that is always the way, isn't it.

I love that description of knowledge as being like a spotlight on the floor of a large dark area. The illuminated area is what you know and the darkness outside it is what you don't know. The edge of the circle represents the questions you have. As you enlarge your knowledge, so your questions grow. A lovely paradox.