In the case of the electrons in the atom,those at lower levels of potential energy(lower shells, or lowern) are more stable and less easily disrupted than those at higher levels of potential energy.Chemical reactions are fundamentally electron transfers between atoms.In a chemical reaction,it is the electrons in the outermost shell that react,that is to say,get transferred from one atom to another.That’s because they are the most easily disrupted, or the most available for reactions.The outermost shell is the marketplace where all electron trade takes place. Accordingly,it has a specail name.It is called the velence shell.