Why the bond angle of water is 105 instead of 109?

In summary, the electron arrangements in central oxygen are not completely evenly distributed, which causes the bond angle between the hydrogens to decrease. This is due to the extra concentration of negative charge in the vicinity of the oxygen, which pushes the bonds together slightly.
  • #1
scientist91
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Answer please.
 
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  • #2
The outer shell electrons in the central oxygen are not completely evenly distributed. There's 4 'pairs', two of which are entirely the oxygens while the other two pairs the oxygen has to share with the two hydrogens it's bonded to (at least the model is kinda like that).

As such there's a slight redistributing of the charges around the oxygen with the two unbounded pairs being a bit higher in electron charge density than the areas where the hydrogens connect to the oxygen. Hence the bonds are pushed together ever so slightly by this extra concentration of negative charge and you get a decrease in the bond angle of the hydrogens compared to say methane which is an evenly distributed tetrahedron of electrons.

I know that's very much 'high school chemistry' take on the electron arrangements etc but I doubt the answer requires a more 'quantum' approach than that.
 
  • #3
According to Valence Shell Electron Pair Repulsion (VSEPR), a pair of electrons takes more "space" than the usual bonding pair of electrons.
 
  • #4
Shadowz said:
According to Valence Shell Electron Pair Repulsion (VSEPR), a pair of electrons takes more "space" than the usual bonding pair of electrons.

As for a very simplistic answer, this is very correct.
 
  • #5
Shadowz said:
According to Valence Shell Electron Pair Repulsion (VSEPR), a pair of electrons takes more "space" than the usual bonding pair of electrons.
Why when there are in the bond pair also 2 electrons and the lone pair 2 electrons. What is the problem? Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.
 
  • #6
scientist91 said:
Why when there are in the bond pair also 2 electrons and the lone pair 2 electrons. What is the problem? Also the sp3 hybrid orbitals are on same energy level so, they are all similar by size.

When unshared electron (electrons in lone pairs) are present, VSEPR does "predict" slight distortion of ideal bond angels. It is assumed that an outer atom "pulls out" in bonding electron pairs and makes them require less space about the central atom than the unshared electrons.
(1) The bond angle tends to distort to give more room ("space") to unshared electron pairs.
(2) in geometry with positions that are not equivalent, the unshared electron pairs will occupy the more spacious positions.

In other words, if there is one only one unshared pair electrons, that pair electron will not affect the geometry of the compound. For example, SnCl2 and BF3 have the same geometry, the angle between the atoms are 120 degree.
However, if there is more than 1 unshared pair of electrons, these pairs will try to locate as fas as possible from each others. That explained why H2O and CF4 are different from geometry.

I agree that H2O and CF4, for example, are sp3 hybridization. But the difference in shape (geometry) has nothing to do with the energy level when you comparing these 2 compounds. But if you compare the geometry of H2O when the bond angles are all 109.5 degree with the one that has less than 109.5 degree, then we will say that the one that has the bond angle less than 109.5 is more stable which means it's in the lower energy.

Also, VSEPR is a theory, so at least, we should not try to prove that it wrong! Some days, when you become a chemist, you can work on that and give another theory which shows that unshared pair electrons take less space than bonding pair electrons. But up until now, we agree with this theory and it helped us explain not only the stuff about geometry of compound, but also some mechanism in organic chemistry.

Have fun studying!
 
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FAQ: Why the bond angle of water is 105 instead of 109?

What determines the bond angle in water?

The bond angle in water is primarily determined by the shape of the molecule and the electron pair repulsion. Water has a bent molecular shape due to the presence of two lone pairs of electrons on the oxygen atom, which influence the bond angle.

Why is the bond angle in water not 109.5°?

The bond angle in a water molecule is not 109.5°, which is the tetrahedral angle, because of the two lone pairs of electrons on the oxygen atom. Lone pairs repel more strongly than bonding pairs, causing the hydrogen atoms to be pushed closer together, reducing the bond angle to approximately 105°.

What is the VSEPR theory and how does it explain the bond angle in water?

The Valence Shell Electron Pair Repulsion (VSEPR) theory explains that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. In water, the two lone pairs on oxygen push the hydrogen atoms closer, resulting in a bond angle less than the tetrahedral angle.

How does the presence of lone pairs affect the bond angle?

Lone pairs occupy more space around the central atom than bonded pairs. In water, the two lone pairs on oxygen exert greater repulsion on the bonded hydrogen atoms, reducing the H-O-H bond angle from the ideal tetrahedral angle.

Are there any other factors that influence the bond angle in water?

While the primary factor is the repulsion between electron pairs, the bond angle can also be slightly influenced by the electronegativity of the atoms and the size of the orbitals involved.

Does the bond angle of water affect its properties?

Yes, the bond angle of water significantly affects its properties, such as polarity and hydrogen bonding capability. These properties are crucial in determining water's unique characteristics like its solvent capabilities and surface tension.

Is the bond angle of 105° in water the same under all conditions?

The bond angle of water is approximately 105° under standard conditions, but it can vary slightly under different temperatures and pressures, as well as in different physical states or chemical environments.

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