Reaction with water

Beryllium chloride reacts vigorously and exothermically with water with the evolution of acidic, steamy hydrogen chloride gas. This is typical of covalent chlorides.

In the first instance, it reacts to give hydrated beryllium ions, [Be(H2O)4]2 , and chloride ions.

But the hydrated beryllium ions (called tetraaquaberyllium ions) are quite strongly acidic. The small beryllium ion at the centre attracts the electrons in the bonds towards itself, and that makes the hydrogen atoms in the water even more positive than they usually are. If the solution is hot and concentrated (as it is likely to be if you add water to solid beryllium chloride - a very exothermic reaction), chloride ions can remove one or more of these hydrogen ions to produce hydrogen chloride gas.

All the other ionic chlorides in Group 2 dissolve in water without any obvious reaction.

Note: There is actually a very small amount of reaction between anhydrous magnesium chloride and water, although you wouldn't notice it. Magnesium chloride isn't quite as purely ionic as we sometimes pretend!

Follow this link if you are interested in exploring the naming of complex ions.

. . . or this one for detailed explanations of why complex ions similar to the beryllium one are acidic.

The structure of beryllium chloride

As a gas . . .

Beryllium chloride, BeCl2, is a linear molecule with all three atoms in a straight line. Showing only the outer electrons:

Beryllium chloride is known as an electron-deficient compound because it has the two empty orbitals at the bonding level.

As a solid . . .

If it had this same simple structure as a solid, you would expect the melting point to be much lower than it actually is. It is a very small molecule, and so the intermolecular attractions would be expected to be fairly weak.

Note: If you aren't sure about intermolecular forces (Van der Waals forces) you could follow this link to find out more about them.