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]

This is why...

In the water molecule H2O...The result is a distorted tetrahedral arrangement in which the H—O—H angle is 104.5°.

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]

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]

With water denser than ice,

 

:lol:

 

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!

[LJP07]

Electronagetivity, electron pairs, tetrahedrals etc. are all secondary!

 

So can you give a primary reason except for the above. I believe the above can only be used to describe it in terms of it's structures etc. There is no primary reason from what I know?:)

[HydrogenBond]

Electronegativity is the fundamental reason for the shape of water. Oxygen is more electronegative than hydrogen. This higher electronegativity is implicit of oxygen receiving more stability by gaining extra electrons. This is due to increased magnetic addition. For example, opposite spin electrons magnetically attract inspite of charge repulsion. In the case of oxide or O-2, with the 2P orbital full, these additive pairs will also magnetically add in 3-D via the x,y,z lobes of the P-orbital. In other words, the EM force is both electrostatic and magnetic, higher electronegativity measures an atoms ability to gain magnetic attraction to compensate for the charge repulsion that will occur by gaining an extra electron. The more magnetic addition the higher the electronegativity.

 

In H20, the oxygen is trying to place its electrons in such as way as to maximize its magnetic addition. The hydrogen has its own agenda but does not get as much stability from magnetic addition as can oxygen. It can only mag-add via spin addition between two electrons and not in x,y,z 3-D like the P-orbital electrons around oxygen. Based on the magnetic compromise, the result is the shape we see.

 

If we look at hydrogen bonding it is often viewed as an electro-static bond between H and O. This is only half the story, since the oxygen is quite stable holding extra electrons (O-2) due to mag-add. The attachment of hydrogen, via the hydrogen bond, may lower the charge repulsion for oxygen, but it takes something away from its optimized mag-add, with the result the oxygen gaining potential. Left to its own devices, the oxygen will take the charge repulsion because it offers mag-stabiltiy. The hydrogen bond lowers its net staiblity.

 

The way the oxygen lowers its potential back is via the partial covalent nature of hydrogen bonding. This allows delocalization of the charge attracted electron density that hydrogen takes, allowing oxygen to regain some of the magnetic addition.

 

This battle between hydrogen and oxygen is what makes water so unique. The hydrogen is placed in a state of perpetual potential due to the higher electronegativity of oxygen. The hydrogen keeps trying to lower potential via hydrogen bonding, with the oxygen using its higher electronegativity to make this less than fully satisfying for the hydrogen.