Found some explanation:
Ionic bonding
Consider now a more difficult situation, one in which two different atoms compete for electrons. One example would be the case involving a sodium atom and a chlorine atom. If these two atoms come close enough to each other, both nuclei pull on all electrons of both atoms. In this case, however, a very different result occurs. The chlorine nucleus has a much larger charge than does the sodium nucleus. It can pull on sodium's electrons much more efficiently than the sodium nucleus can pull on the chlorine electrons. In this case, there is a winner in the battle: chlorine is able to pull one of sodium's electrons away. It adds that electron to its own collection of electrons. In a situation in which one atom is able to completely remove an electron from a second atom, the force of attraction between the two particles is known as an ionic bond.
Read more: Chemical Bond - examples, body, used, water, type, form, energy, system, oxygen, parts, History, Covalent bonding, Ionic bonding, Electronegativity, Polar and nonpolar bonds
http://www.scienceclarified.com/Ca-Ch/Chemical-Bond.html#ixzz0eHsjkOIW