One can explain the behavior of the oxygen compounds of second shell
atoms by imagining what would happen if the positive ions, ,
were dropped into water.
This is only an imaginary experiment, because ,
ions, with their entire second electron shell stripped off, are
too reactive to exist in solution. Nevertheless, the products that
these ions would form with water are what actually are observed
in solution when the oxides of these elements are dissolved in water.
ion in aqueous solution surrounds itself with four water molecules
and exists peaceably as a
ion, as shown.
In an acid solution (excess of protons), beryllium also occurs as
ion. Beryllium is more electronegative than lithium, and the beryllium
ion is more highly charged. It therefore pulls on the lone electron
pairs of water oxygens more than
The bond between
and a water oxygen is mainly electrostatic, but the bond between
and water is partially covalent. As
pulls on the water lone pairs, it weakens the O-H bonds of the water
merely holds on tightly to its four hydrating molecules. In basic
solutions, where protons are scarce, the weakened water molecules
each can release one proton, so the hydrated ion in basic solution
instead of .
ion then is surrounded, not by four neutral water molecules, but
by four negative hydroxide ions. The cluster, or complex ion, has
a negative charge. At intermediate acidities, less than four of
molecules can lose protons.