Ionic bonds form between two elements that have a very large difference in electronegativity ie. a metal and a non-metal. The electronegative element will want an electron to make it stable whilst the less electronegative element wants to donate an electron to make it stable. Therefore the metallic element donates a electron to the non-metal and both become ions, which attract each other. Eg. Na becomes Na+ and Cl becomes Cl-, the opposite charges attract, forming the bond.
