Why is the Shape of Hydrogen Selenide (H2Se) Bent?

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Discussion Overview

The discussion centers around the molecular geometry of hydrogen selenide (H2Se), specifically why it adopts a bent shape. Participants explore the implications of valence electrons, lone pair repulsion, and electron pair geometry in relation to bond angles.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that H2Se has 6 valence electrons, with 2 used for bonding with hydrogen atoms and 4 remaining as lone pairs, suggesting that this configuration leads to a bent shape.
  • One participant argues that the repulsive forces between the lone pairs are greater than those between the bonding pairs, which causes the bonding pairs to be pushed closer together, affecting the bond angle.
  • Another participant questions the logic of pushing bonding pairs closer if the lone pairs are already arranged at 180° to each other, implying that this arrangement should maximize distance.
  • A later reply references a diagram to illustrate how the arrangement of bonds and electron pairs can maximize distances, suggesting that the 3D perspective is important for understanding the geometry.
  • Some participants compare H2Se to water (H2O), noting that while the electron geometry is similar, the angles and lengths differ.

Areas of Agreement / Disagreement

Participants express differing views on the implications of lone pair repulsion and the resulting bond angles. The discussion remains unresolved, with no consensus on the exact reasoning behind the bent shape of H2Se.

Contextual Notes

Participants do not fully agree on the effects of lone pair repulsion versus bonding pair arrangement, and there are unresolved aspects regarding the interpretation of molecular geometry diagrams.

coconut62
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Why does H2Se have a bent shape?

It has 6 valence electrons. 2 are used to bond with each of the hydrogens. 2 more up and 2 more down. So the repulsion of the lone pairs cancel off each other. Isn't this arrangement going to make the bond angle 180°?
 
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coconut62 said:
Why does H2Se have a bent shape?

It has 6 valence electrons. 2 are used to bond with each of the hydrogens. 2 more up and 2 more down. So the repulsion of the lone pairs cancel off each other. Isn't this arrangement going to make the bond angle 180°?

The repulsive forces between the lone pairs is greater than the forces between the 2 bonding pairs so the bonding pairs get pushed closer to increase the distance between the 2 lone pairs.

Another way to look at it is that molecules with 4 pairs of electrons form a tetrahedral shape, think methane, except in molecules like H2O or H2Se, 2 of the corners do not have another atom but they do have an electron pair.
 
trollcast said:
The repulsive forces between the lone pairs is greater than the forces between the 2 bonding pairs so the bonding pairs get pushed closer to increase the distance between the 2 lone pairs..

Push closer to where? If the two lone pairs are arranged at 180° to each other and so does the bond pairs, then maximum distance will be achieved, no?
 
coconut62 said:
Push closer to where? If the two lone pairs are arranged at 180° to each other and so does the bond pairs, then maximum distance will be achieved, no?

http://www.elmhurst.edu/~chm/vchembook/images/206water.gif

Try looking at that and see how the arrangement of the bonds and electron pairs maximises the distances between them (I think you are maybe only thinking about moving them round in the same plane)

I know the diagram says water but H2Se has the same electron geometry just the angles and lengths are a bit different.

Ps. The perspective of that diagram isn't great since it appears the lone pairs are closer together than the bonding pairs but in reality the bonding pairs should be closer but it should show the sort of 3d arrangement of the pairs around the central atom.
 
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