Why Does Salt Dissolve? Atoms That Swapped an Electron

Some elements hold their outer electrons loosely and others attract additional ones strongly, and when the two meet an electron may transfer completely rather than being shared. The atom losing an electron becomes positively charged and the one gaining it becomes negative, and the resulting ions attract each other electrostatically in every direction rather than along a particular line. That lack of direction is the defining feature, since the attraction is not a bond between two specific particles but a general pull among all of them, so the ions pack into a regular three-dimensional arrangement in which each is surrounded by as many oppositely charged neighbours as geometry allows. A crystal of such a compound is therefore a single extended structure rather than a collection of molecules.

The properties of ionic compounds follow directly from the structure:

• •High melting points, since breaking the structure means overcoming attractions throughout it
• •Hardness, for the same reason
• •Brittleness, since a blow that shifts one layer brings like charges together and splits the crystal
• •No conduction when solid, because the charged particles are locked in place
• •Conduction when molten or dissolved, because the ions become free to move
• •Solubility in water and poor solubility in non-polar solvents

Dissolving requires overcoming the attractions holding the crystal together, and water manages it because its molecules are electrically lopsided, with a partial negative charge at the oxygen and partial positives at the hydrogens. Those molecules cluster around each ion at the crystal surface with the appropriate end facing it, and the energy released as they do so is comparable to the energy required to remove the ion from the lattice, which is why the process happens at all. Whether a particular salt dissolves depends on which of those two quantities is larger, which is why some are extremely soluble and others practically insoluble despite identical bonding. The dissolved ions remain surrounded by water molecules, which is why an ionic solution conducts and why the ions behave differently in water than they do dry.

The quantity determining how strongly an ionic solid is bound is the energy required to separate the whole crystal into isolated gaseous ions, and what controls it is straightforward. Higher charges bind far more strongly, so a compound of doubly charged ions is bound several times more tightly than one of singly charged ions, which is why some salts melt below a thousand degrees and others require twice that. Smaller ions sit closer together and bind more strongly for the same charge. The arrangement of the packing contributes a factor that depends only on the geometry. That quantity cannot be measured directly, since nobody can produce a gas of separated ions and watch them combine, so it is obtained indirectly by combining measurable quantities in a cycle, which is a standard method and a good illustration of how inaccessible quantities are pinned down.

Treating bonding as either transfer or sharing is a simplification that teaching relies on and reality does not respect. The extent to which an electron actually transfers depends on how strongly the two atoms attract electrons, and the difference between them varies continuously, so real compounds sit along a range from almost complete transfer to almost equal sharing. Compounds conventionally called ionic retain some sharing, and covalent bonds between unlike atoms are polarised, meaning the electron pair sits nearer one atom. Small highly charged positive ions distort the electron cloud of large negative ones enough to introduce substantial covalent character, which explains why several compounds classed as ionic behave in ways the simple picture does not predict. The categories remain useful and should be held loosely.

A transferred electron leaves two oppositely charged ions attracting in every direction, so the crystal is one extended structure rather than a set of molecules. Brittleness follows because shifting a layer brings like charges together, and conduction requires melting or dissolving so the ions can move. Water dissolves them because its lopsided molecules cluster around each ion and release comparable energy.