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miguel asked in 科學及數學化學 · 1 decade ago

Bonding between H+ & OH-ions

During neutralization, the H+ ion of the acid and OH- ion of the base would combine to form H2O, i.e. water.

My question is, why, then, is water a covalent compound? Why is the compound formed by H+ and OH- is covalent bond instead of ionic?

1 Answer

  • 1 decade ago
    Best Answer

    lets compare H+ and Na+.

    H+ is much more energetically stable than Na+.

    Both of them carry +1 charge. however, the effective nuclear charge of H+ is infinitely high (note that there is no electron in H+) that polarizes by induction OH- in a so great extent so that one of the lone pair on the oxygen atom is distorted enormously between OH- and H+ and the nature of electrostatic attraction between H+ and OH- predominantly involves electron sharing (datively from OH-) rather than electron transfer. in this case, covalent bond is a term to describe the nature of O-H bond in H2O more reasonably. therefore, the imaginary ionic compound H+OH-(s) is not energetically stable at standard condition.

    even if in aqueous solution, H+ is not stable to exist. a complex [H3O(H2O)5]+, or simply written in H3O+, is usually formed instead reducing the charge density of H+. upon this, the formation of [H3O(H2O)5]+ is exothermic as electrical PE is coverted to thermal energy when H+ is solvated. this can be verified by large amount heat is formed when a conc. strong acid like H2SO4 is added into water.

    on the other hand, the only outermost electron of Na is sufficiently shielded by the inner shell. and the charge density is not high. the electrical PE involved when the outermost electron is brought to infinity. thus, an electron is readily donated from Na. in order words, Na+ is energetically feasible to form.

    in addition, Na+ has an octet eletronic configuration which gives itself energetical stability.

    therefore, Na+ and OH- do not form a covalent compound but instead an ionic one since Na+ is stable and OH- is not much distorted.

    this is consistent with the comparision of electronegativites between Na or H and O as electronegativity describes the relative ability of an atom to bind the bonding electrons.

    actually, ionic bond and covalent bond is the same matter about the electrostatic attraction between the both nucleus towards their electrons but of difference polarity. ionic bond has the highest polarity while covalent bond has the lowest. from ionic to covalent nature, the change of polarity within a bond is gradual.

    furthermore, why doesn't H behave like alkali metals, Na, K, ... having a giant metallic structure with one valence electron?

    due to the above reason, outermost electron of those metals are losely held and delocalization of electrons by neighbouring lattice ion is possible.

    on the other hand, delocalization of electrons involves high PE in an imaginary giant metallic structure of H since repulsion between H+ lattice ion and instantaneous average electron-nucleus separation make this structure impracticable.

    in conclusion, H has totally different from alkali metals and hence H is seldom regarded as group I element though it has one outermost shell electron.

    Source(s): myself
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