Abstract: The water molecule, H₂O, can attach a hydrogen ion (H⁺) perhaps from a dissolved acid, to become a hydronium ion, H₃O⁺, sometimes called a hydroxonium ion. This ion, if surrounded by water molecules, can form hydrogen bonds with them. The question is what is the molecular structure of the resulting hydronium ion hydrate? The mind conceives two serious possibilities. But what do studies reveal?
It is a general principle of nature that a system is more stable if charges are maximally dispersed. In the field of chemistry, the best known example of this involves the polar solvent water. Although the sum total electric charge of a water molecule is zero, an axis drawn along the length of the molecule reveals partial positive and negative charges on opposing sides of the axis. Water, after all, is not a linear species; it is bent 104.45º (see image).
The positive charge of a hydronium ion is drawn to the partial negative charge of a water molecule. The end result is possibly a full chemical bond, and at least a hydrogen bond can be formed between the hydronium ion and the molecule of water to form a hydronium ion hydrate. Such a structure is seen in the image below.
However, a hydronium ion possesses a full charge and may draw a number of water molecules to itself. In fact, it is considered a very real possibility that a single hydronium ion may draw as many as 20 molecules of water to itself. Other conceptualizations suggest a number of about 5 or 6. To spread a lone charge out over so many atoms is truly a stabilizing dispersion of charge!
One of particular interest is the Eigen concept. In it, the hydronium ion is encased by three water molecules, H₉O₄⁺. Again, note the image. One additional thought. In the image, imagine the structure from a horizontal position with the central oxygen at top and the outermost hydrogen atoms of the water molecules assuming the lowest positions. That would give the eigen hydronium ion the shape like the upper half of a skull.
While the smaller structures appeal to the human desire for simplicity, and sometimes they are the right choice, the principle of the spread of charge appeals to reckoning a higher level of water association. At the same time, why must a single structure be the only possible answer? Could some of the hydrogen bonds become full bonds, while some of the full bonds diminish or downgrade to hydrogen bonds?
Indeed, could not a hydronium ion, in a sea of water molecules, make and break connections much like Tarzan swinging between grapevines travels through the jungle. That is, why could not a variety of structures prove to be the true solution?
Note: You might also enjoy “What is a Hydronium Ion?”
- Water Structure and Science – by Martin Chaplin: Hydrogen Ions
- Springer Nature Chemistry: Direct Observation of a Hydronium Ion Coordination by a Proton