Why do the Na ions safely reside in the water whereas Na does not?
Elemental Na reacts with water to become Na+ ions. The reaction is:
Na + H2O --> Na+ + OH- + (1/2)H2 + energy
The products have a much lower energy state than the reactants. This is
why the reaction is so exothermic that the hydrogen gas often catches
fire. The Na+ ions, in a much lower energy state than the elemental Na,
will not react with water (or much of anything else).
In anthropomorphic terms, the elemental Na "wants" to be Na+ ions (so it
can have a noble-gas-like electron configuration). Having its desire
met, the Na+ ions are happy to stay as they are.
To answer the original question (does saturated salt water have NaCl
molecules in it?), NaCl never exists as molecules. In solid NaCl, the
positive Na and negative Cl ions form a lattice where each positive Na
ion is surrounded by 6 negative Cl ions (and each Cl ion is surrounded by
6 Na ions). In order for there to be NaCl molecules, a single Na must be
bonded to a single Cl so that they form a pair. In the salt crystal,
each Na is equally attracted to 6 separate Cl ions which are, in turn,
equally attracted to other Na ions, etc. No discrete molecules.
Water molecules, on the other hand, are discrete units. We can say that
these two hydrogen atoms and this one oxygen atom form a group (a
molecule). Hydrogen bonding makes things a little more complicated than
my simple answer, but the essence of molecules is that they are groups of
bonded atoms that are distinct and separate from neighboring molecules
(whether solid, liquid, gas, or in solution).
David J. Hamilton, Ed.D. * And gladly woulde he learn,
Portland Public Schools * and gladly teche.
djhamil@teleport.com * - Chaucer