Answer: The electrons located on the outer shells of an atom are the ones responsible for that atom's chemical properties. These electrons interact with other atoms and are either shared (covalent bond) or given up (ionic bond) to the second atom. This is the basis for chemical bonding.
Explanation: All atoms have electron orbitals that define the locations and energy levels for each electron associated with an atom. While the ones closer to the nucleus are generally secure and tightly bound, those in the higher energy shells, further from the nucleus, will interact with neighboring atoms that have enough kinetic energy to come close enough for each atom's outer electrons to take on new configurations that allow sharing, or outright theft, of one another's electrons.