In order for an ionic bond to form, the beryllium has to let go of its electrons. It is too electronegative to do that.

Note: The trends in electronegativity in Group 2 are discussed on another page. That page looks at the way the electons are arranged in the beryllium-chlorine bond compared with the magnesium-chlorine bond.

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Beryllium forms 4-coordinated complex ions

Some simple background

Although beryllium doesn't normally form simple ions, Be2 , it does form ions in solution. In these, the beryllium ion becomes attached to four water molecules to give a complex ion with the formula [Be(H2O)4]2 .

The ion is said to be 4-coordinated, or to have a coordination number of 4, because there are four water molecules arranged around the central beryllium.

Many hydrated metal ions are 6-coordinated. For example, magnesium ions in solution exist as [Mg(H2O)6]2 .

The water molecules in these ions are attached to the central metal ion via coordinate bonds (dative covalent bonds). One of the lone pairs on each water molecule is used to form a bond with an empty orbital in the metal ion.

Each time one of these bonds is formed, energy is released, and the ion becomes more stable. It would seem logical for the metal ion to form as many bonds like this as it possibly can.

Note: If you aren't happy about coordinate bonding you must follow this link before you go on. You will find the bonding in hydrated metal ions discussed in some detail on that page.

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Why does beryllium only attach four water molecules?

The hydration of beryllium

The problem is that there has to be somewhere that the lone pairs on the water molecules can attach to. Beryllium has the electronic structure 1s22s2. It is helpful to draw this as an "electrons-in-boxes" diagram: