How do atoms become stable and nonreactive

All I need, is what the question says. Thank you all!

Atoms become nonreactive when they have filled their outer electron shells. The closer to filling the shell it gets, the more reactive it gets. The mid-way point is pretty non reactive too.

Atoms become stable when they undergo decay. There is an excellent website about this. This link will take you to their glossary. Search for Alpha decay, Beta decay, Electron capture and Proton emission.

http://education.jlab.org/glossary/index.html

When they fill out their "valence electron shell", or the outer most orbital in the electron cloud. Noble gases are nonreactive because their electron shells are filled completely. That is also the explanation for diatomic elements. :)

They became stable and non reactive when they are not used frequently.