Ionic bonding is
the complete transfer of valence electron(s) between atoms. It is a type
of chemical bond that generates two oppositely charged ions. In ionic
bonds, the metal loses electrons to become a positively charged cation,
whereas the nonmetal accepts those electrons to become a negatively
charged anion. Ionic bonds require an electron donor, often a metal, and
an electron acceptor, a nonmetal.
bonding is observed because metals have few electrons in
their outer-most orbitals. By losing those electrons, these metals can
achieve noble gas configuration and satisfy the octet rule. Similarly,
nonmetals that have close to 8 electrons in their valence shells tend to
readily accept electrons to achieve noble gas configuration. In ionic
bonding, more than 1 electron can be donated or received to satisfy the
octet rule. The charges on the anion and cation correspond to the number
of electrons donated or received. In ionic bonds, the net charge of the
compound must be zero.
Please refer to http://chemwiki.ucdavis.edu/Organic_Chemistry/Fundamentals/Ionic_and_Covalent_Bonds for more details.