There are eight electrons needed for a full shell.
Atoms are most stable and least reactive when their outermost electron shells are full. Most of the elements important in biology require 8 electrons to be stable in the outermost shell, a rule of thumb known as the octet rule. Their outermost shells can only hold 8 electrons, so there are only 8 elements in both periods.
The full valence shell is the most stable electronic configuration. Other groups of elements have partially filled valence shells and gain or lose electrons to achieve stable electronic configurations. Atoms can gain or lose electrons to achieve the most stable electronic configuration, the full valence shell. Other groups of elements have partially filled valence shells and gain or lose electrons to achieve stable electronic configurations.
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