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by HIGHLYTOXIC
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HIGHLYTOXIC
#1
Aug11-04, 02:00 AM
P: 48
If we take a look on the following compounds in the Oxygen family, the bond angle increases as we go down the group as :

H2O > H2S > H2Se

Whats the reason for this increase?

Is that :

1. Oxygen is more electronegative than the two, so it holds its lone pairs more tightly than the other two..Thus, the lone pairs are tightly bound to the oxygen atom, not free to wander about & repel the bond pair with hydrogen, therefore making the angle much larger.

OR

2. Oxygen being more electronegative, pulls the two bond pairs close to itself, as the bond pairs move closer, the distance btw them decreases & this makes the bond angle larger.

Which of the above reason is right?

Thanks!
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fawaz
#2
Aug18-04, 07:57 AM
P: 8
oxygen being electro negative..it holds the two bond pairs more tightly than the other other elements.. as the are pulled towards oxygen ,repulsion between these pairs increases.hance they posses a larger bond angle

the second one is correct
Gokul43201
#3
Aug18-04, 10:14 AM
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I believe there is also a steric effect coming from the size of the anion. Comparing ionic radii, O(2-) < S(2-) < Se(2-). So the bond pairs (H-atoms) are closer to each other in H20 giving rise to stronger repulsion, and hence greater bond angle.

maverick280857
#4
Aug19-04, 12:57 AM
P: 1,780
Have a question.

The primary reason is that electronegativity decreases down the group as you go from O to S to Se. So the attraction for the two bonded pairs of electrons decreases as well. As a result, the electron pairs are not pulled as strongly by the central atom as you go down the group causing and therefore, electron pair repulsion decreases, decreasing the bond angle.

The electronegativity reason is more pronounced than the effect of increasing size of the central atom, as we were taught in class. However, size does have some role to play so we must not rule it out of our thinking completely. The second one is therefore correct and holds in general for most such bond angle predictions Highlytoxic.

Hope that helps,

Cheers
Vivek


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