What does it mean to say that some elements are reactive and form ions easily whereas others do not?

 As ions are formed when atoms gain an electric charge .
cations (ions with +ve charge) and anions (ions with  -ve charge). Others are more stable when they are together than when they are apart, so they will stay together.hope this helps

The nucleus in these elements has a weaker hold on the outer most electron due to electron shielding phenomenon. Thus the outer most electrons tend to escape making the atom more reactive. Basically, these elements can easily give away or lose electrons in the outer shell to other atoms so that they can form linkages