Ions are charged atoms or molecules. They may have a plus charge, a minus charge or even both. Examples of each are the positive sodium ion (Na+1), the negative bisulfate ion (HSO4-1) and the glycine zwitterion (H3N+1‐CH2‐C(O)2-1).
Charge & Nature
Even as “nature abhors a vacuum,” it likewise abhors a concentrated electrical charge – lightning well illustrates the point. Although ordinary table salt exists in water solution as charged ions, those ions are not isolated as the above shorthand symbols would indicate. The ions are stable in water because the charge is spread out or diluted, as it were. Consider what happens when a crystal of the substance is placed in a beaker full of water.
Water molecules (H2O) behave as if they have a small charge, which, in fact they do. This is partly because an oxygen atom of water does not have its hydrogen atoms attached at opposite sides. Rather, looking a bit like the head of a mouse with its two small ears, they are off center. The side of the water molecule with its center between the two hydrogen atoms carries a small positive charge. The other side – away from the hydrogen atoms – carries an equivalent, small negative charge. Thus a water molecule has two poles, or oppositely charged centers.
Dissolving an Ionic Solid
Some water molecules tend to point their negative side toward sodium atoms at the crystal surface. Other water molecules join in with their positive or hydrogen atom-bearing sides tending to line up with electron-rich chlorine atoms. These fluid water molecules pull at these atoms, successfully breaking them away from the crystal. Water molecules surround the ions.
Interestingly, a sodium ion surrounded by water molecules points the hydrogen atoms outward to form a kind of cage of hydrogen. The positive charge can be shared with each and every one of these hydrogen atoms, so the charge per hydrogen is really quite small. Charge spread stabilizes the ion.
A similar even though seemingly opposite event occurs in the case of chlorine. Since the chloride ion is negative, it draws the positive, hydrogen-rich part of a number of water molecules, exposing only the hydrogen-poor oxygen ends outward, as its charge cage. The oxygen atoms each can carry some of the chlorine ion’s negative charge, so the charge per atom is really quite small. Again, charge spread stabilizes the ion.
Organic Chemistry – Hydroxyl versus Water as Leaving Group
Many organic chemistry reactions involve ions. Sometimes the ion is a positive carbocation (also called a carbonium ion). Depending upon the structure, such ions, with their charge on carbon, may rearrange, putting the charge on a hydrogen atom within the molecule. Quite generally, it is not a positive hydrogen atom or a negative hydroxyl ion that leaves in most organic reactions – they are termed “poor leaving groups.” The explanation as to why may be given in terms of charge and acidity or basicity.
Consider a reaction example: the bromination of methyl alcohol. Not considering the reaction mechanism, the overall reaction is CH3OH + H‐Br → CH3Br + H‐OH. The question is, does hydrobromic acid break apart and the bromide ion react in this fashion: CH3OH + Br-1 → CH3Br + OH-1? No, it does not. Instead, the reaction is said to proceed in this fashion: H‐Br + CH3OH → Brδ—CH3–OH2δ+ → CH3Br + H2O, where the delta refers to partial charge.
Notice that the positive charge on the leaving group is spread out over two hydrogen atoms, until it disappears altogether. This is another example, albeit somewhat disguised, in which charge spread stabilizes an ion.
References & Resources:
ACS Publications: Structure and Bonding of the Sulfamic Acid Zwitterion
“Essentials of Organic Chemistry…,” page 188: Leaving Groups by Paul M. DeWick.