Water

[Little Bang]

Can anyone give me the current explanation of why there is a 104.45 degrees between the hydrogen bonds of water?

[Michaelangelica]

Ask
HydrogenBond he is sure to know.

[Monomer]

This is why...

In the water molecule H2O, the single electron of each H is shared with one of the six outer-shell electrons of the oxygen, leaving four electrons which are organized into two non-bonding pairs. Thus the oxygen atom is surrounded by four electron pairs that would ordinarily tend to arrange themselves as far from each other as possible in order to minimize repulsions between these clouds of negative charge. This would ordinarly result in a tetrahedral geometry in which the angle between electron pairs (and therefore the H-O-H bond angle) is 109°. However, because the two non-bonding pairs remain closer to the oxygen atom, these exert a stronger repulsion against the two covalent bonding pairs, effectively pushing the two hydrogen atoms closer together. The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°.

Water and its structure

[Larv]

Originally Posted by Monomer

This is why...

From: Water and its structure

Over what lapse of time do you suppose this water tetrahedron actually exists geometrically? Maybe liquid water never attains such a geometry (in a temporal sense), but I'm not entirely sure. Am I right or wrong?

—Larv

[Mercedes Benzene]

Over what lapse of time do you suppose this water tetrahedron actually exists geometrically? Maybe liquid water never attains such a geometry (in a temporal sense), but I'm not entirely sure. Am I right or wrong?

Why would it NOT attain the geometry that it does??
And a water molecule IS NOT a tetrahedron.

[Larv]

Originally Posted by Mercedes Benzene

Why would it NOT attain the geometry that it does??
And a water molecule IS NOT a tetrahedron.

Because it's an electromagnetic thing that doesn't fixed a geometry?
However, like you, I don't think it is a tetrahedron, if there is a fixed geometry. My gross model of a water molecule is a softball with two dimes stuck to its surface, separated by 104.5 degrees of the softball's circumference. My understanding of this water-molecule model is that the dimes are never in a fixed location with respect to the softball, but rather "floating" on its electron clouds.

—Larv

[HydrogenBond]

The angle discussed is true for an isolated water molecule. The unbonded electrons repel each other, forcing the hydrogen to get a little closer than a tetrahedron. The extra strain put on the hydrogen is a reflection of the higher electronegativity of oxygen. Oxygen wants the electrons more and isn't going to make it easy for hydrogen.

When you talk about hydrogen bonding angle, this is different, since it involves more than one water molecule. The tetrahedral of OH- bonds does form in some versions of ice because water will expand as ice. The hydrogen gets the better tetrahedral angle but loses something by being pushed further away during expansion. Oxygen is still being stubborn with electrons, so it gives and takes.

The thing about hydrogen bonds is that a hydrogen bond is maximized if it form a nice straight line with the shared electrons on another water. 180degrees. The tetrahral in ice allows all the hydrogen bonds to line up in straight lines. Beiing pushed away to reflect expansion, puts them at the correct distance for partial covalent bonding to also form. This is how the oxygen gets even and assert its higher electronegativity.

In liquid water, the tetrahedral state is not as common. Water forms all type of random and extended structure. But hydrogen just can't stay very long in the tetrahedral. With water denser than ice, the oxygen can't push hydrogen away for the tetrahedral to be stable, so hydrogen assumes placement that is less then a perfect tetrahedral. What this does, is make the average hydrogen bond angle with the shared electrons different than 180, with bond strength falling off rapidly with deviation from linearity. This is another trick by oxygen to assert its higher electronegativity.

[freeztar]

Originally Posted by HydrogenBond

With water denser than ice,

I thought solid form was always more dense than liquid...
But I guess it does make sense if you consider the strange fact that ice expands. Am I thinking right on this?

[LJP07]

Generally, when you take into account the electronegativity mentioned above, normally, in compounds like Boron Trichloride and Ammonia, Water etc, you would take 2.5 ( or 2.75, normally 2.5) degrees approximately for each lone pair that exists, so whereas the normal bond angle is 109 Degrees, the two lone pairs ( 4 electrons not involved in bonding ) make this 104 degrees approx. This gives a distorted tetrahedral or Pyramidal Shape to the molecule.

[hallenrm]

The primary reason why scientists believe that the angle between two O-H bonds in a water molecule is 104.5 degrees is that only if we believe so that we can explain various observed properties of water. Electronagetivity, electron pairs, tetrahedrals etc. are all secondary!