| MadSci Network: Chemistry |
Ionic and covalent bonding are really just models at the extremes of a continuum of types of bonding. Where the dividing line between them lies is arguable, but a generally accepted definition is that if the difference between the electronegativity values of two elements is over 1.7, then the bonding between them is regarded as ionic, otherwise it is covalent. The smaller the difference, the less polar the covalent bond is, until at differences of less than about 0.5 the bonding is considered to be non-polar (pure covalent, if you like).
If your students are not too old (I assume they must be middle school age), then you just need to tell them that between a metal and a non-metal you get ionic bonding, between two non-metals you get covalent, and between metals you get metallic bonding.
Ionic bonding involves complete transfer of the valence electrons from the metal to the non-metal, giving both atoms a stable octet (full outer shell). In covalent bonding, both atoms are in the position of needing an electron or 4 to achieve an octet, so they "share" by a combining of electron clouds known as a covalent bond.
For more details and examples any high school chemistry book should suffice.
Try the links in the MadSci Library for more information on Chemistry.