Why Is the Angle Between Hydrogen Atoms in Water Molecules 105 Degrees?

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Discussion Overview

The discussion centers around the angle between hydrogen atoms in water molecules, specifically questioning why it is 105 degrees and whether this angle would be the same in heavy water molecules. The scope includes theoretical explanations and references to established models like VSEPR theory.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • John questions the reason for the 105-degree angle between hydrogen atoms in water and its potential consistency in heavy water.
  • Another participant suggests that Feynman likely found the topic interesting, implying a subjective perspective on the discussion's significance.
  • John seeks clarification on the angle rather than the interest level of Feynman's discussion.
  • A participant proposes the VSEPR (Valence Shell Electron Pair Repulsion) theory as a possible explanation, noting that the arrangement of electron pairs leads to a deviation from the ideal tetrahedral angle of 109.5 degrees due to repulsion effects.

Areas of Agreement / Disagreement

Participants express differing focuses, with some emphasizing the interest in Feynman's discussion while others seek a more technical explanation. The discussion does not reach a consensus on the reasons behind the 105-degree angle.

Contextual Notes

The explanation involving VSEPR theory introduces assumptions about electron pair interactions and their influence on molecular geometry, which may not be universally accepted or fully detailed in the discussion.

johnhb
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In "Six Easy Pieces", Chapter 1, Richard Feynman discusses how the angle between hydrogen atoms in a water molecule is 105 deg. Why is this? Would it be the same for a heavy water molecule?

John
 
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Probably Feynman discussed it because he found it interesting or illuminating!
 
Thank you, Dr.Dru, but I was wondering why the angle is 105 deg?

John
 
Yes, but what is the problem? I thought Feynman gave an explanation. Is there anything unclear with his explanation?
One of the easiest explanations is the VSEPR (Valence Shell Electron Pair Repulstion) theory.
According to it the free and bound electron pairs repell each other. In H2O there are two free electron pairs and two bound ones, hence they try to arange in a tetrahedral configuration.
The bound electron pairs extend over two atoms, hence the repulsion between them is a little bit smaller whence the angle between the bound pairs is a little bit smaller than 109.5 deg, the ideal tetrahedral angle.
 

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