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I Bond length of a hydrogen bond

  1. Apr 15, 2016 #1
    Hello, in ice I have read that the average bond length of the hydrogen bond is about 2 Angstroms. This is roughly twice the O-H (covalent) bond length. My question is what really determines the length of the hydrogen bond? Why wouldn't the hydrogen be drawn closer to the next water molecule's oxygen: what is the source of repulsion?
    Thanks and apologies for the naïve tone!
  2. jcsd
  3. Apr 15, 2016 #2
    When two atoms come very close, the force between them is always repulsive, because the electrons stay outside and the nuclei repel each other.
    Unless both atoms are ions of the same charge (e.g., both negative) the forces between atoms is always attractive at large internuclear distances r.

    Since the force is repulsive at small r, and attractive at large r, there is a distance at which the force is zero. This is the equilibrium distance at which the atoms prefer to stay.

    The interaction energy is the potential energy between the atoms. It is negative if the atoms are bound and positive if they can move away from each other.
    The interaction energy is the integral of the force over the separation distance, so these two quantities are directly related.
    The interaction energy is a minimum at the equilibrium position. This value of the energy is called the bond energy,

    Bond length is related to bond order: when more electrons participate in bond formation the bond is shorter. Bond length is also inversely related to bond strength and the bond dissociation energy: all other things being equal, a stronger bond will be shorter.
    In a bond between two identical atoms, half the bond distance is equal to the covalent radius.
    for details see https://en.wikipedia.org/wiki/Bond_length
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