Note: If you aren't happy about orbitals you really ought to follow this link before you go on. You may want to explore further in that part of the site as well. Unless you understand exactly what this electrons-in-boxes diagram is about, you won't be able to make sense of what is coming up next.

When beryllium forms a 2 ion it loses the 2 electrons in the 2s orbital. That leaves the 2-level completely empty.

The 2-level orbitals reorganise themselves (hybridise) to make four equal orbitals, each of which can accept a lone pair of electrons from a water molecule. In the next diagram the 1s electrons have been left out. They aren't relevant to the bonding.

Each water molecule, of course, has two lone pairs of electrons. Only one of them is shown to avoid cluttering the diagram.

Notice that once four water molecules have bonded in this way, there isn't any more space available at the bonding level. All the empty orbitals from the original beryllium ion are being used.

The water molecules arrange themselves to get as far apart as possible - which is pointing towards the corners of a tetrahedron. The ion therefore has a tetrahedral shape.

The hydration of magnesium

You might think that magnesium would behave just the same, but at the 3-level there are 3d orbitals available as well as 3s and 3p.

When the magnesium ion is formed, it leaves empty 3s, 3p and 3d orbitals. When that ion is hydrated, it uses the 3s orbital, all three of the 3p orbitals and two of the 3d orbitals. These are reorganised to leave a total of six empty orbitals which are then used for bonding.

Why does magnesium stop at attaching six waters? Why doesn't it use the remaining 3d orbitals as well? You can't physically fit more than six water molecules around the magnesium - they take up too much room.

What about the other ions in Group 2?

As the ions get bigger, there is less tendency for them to form proper coordinate bonds with water molecules. The ions become so big that they aren't sufficiently attractive to the lone pairs on the water molecules to form formal bonds - instead the water molecules tend to cluster more loosely around the positive ions.

Where they do form coordinate bonds with the water, however, they will be 6-coordinated just like the magnesium.

Beryllium hydroxide is amphoteric

Amphoteric means that it can react with both acids and bases to form salts.