Beryllium 9 is the only stable isotope.

The shape shows clearly how it resembles two intertwined Lithium's. These 2 rings of 5 protons are equated with the valence number possibilities. In this case + 2. Beryllium valence value as nuclet in the heavier element tends to be either + 1 or +2  Beryllium itself has all the room needed to make a default connection of +2. In heavier elements the valence number can vary for several reasons such as An extra neutron on an nuclet, a particular chemical combination, or literally missing connection points. The potential of more to be understood here is a real possibility.
Beryllium is in effect a geometric 'in between state' between Lithium and Carbon.