What will atoms with fewer than 8 electrons in their outermost shell do to become stable?

When atoms have fewer than 8 electrons, they tend to react and form more stable compounds.

Atoms will react to get in the most stable state possible. A complete octet is very stable because all orbitals will be full. Atoms with greater stability have less energy, so a reaction that increases the stability of the atoms will release energy in the form of heat or light. Reactions that decrease stability must absorb energy, getting hotter.

Answer: they will go get more from another shell