Final answer:
Most atoms are most stable when they have eight valence electrons in their outer shell, according to the octet rule, which governs their chemical bonding and reactivity.
Step-by-step explanation:
The question pertains to the number of valence electrons that most atoms need to have a complete outer shell and be chemically stable, or "happy". According to the octet rule, the majority of atoms strive to have eight electrons in their valence shell, with the exception of hydrogen and helium, which are stable with two electrons in their outer shell. This rule is critical in understanding the formation of chemical bonds, such as covalent bonds, where atoms share their valence electrons to achieve a full outer shell. For example, oxygen with its six valence electrons will often form bonds that result in the addition of two more electrons, hence reaching the octet and creating molecules like water (H₂O).