Again, the driving principle is the same in either case: to leave an atom with a full valence shell.
First, "Reactivity" depends upon many other factors than the number of electrons in the valence shell.
Period 1 elements follows the duet rule, they only need two electrons to complete their valence shell.
One characteristic that all do have in common is having three electrons in their valence shells.
Each OH- ion has seven electrons in its valence shell.
Some reactive species such as carbenes have incomplete valence shells.
They are always in the last shell of an atom, the valence shell.
The term "valence shell" is a hold over from historical terminology.
A bond is formed when the valence shells of two atoms overlap.
The maximum number of electrons that can occupy a valence shell.
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