Previous Page: Naming Molecules
Now notice the location of these two elements in the periodic table. As a rule, elements to the left (metals) pool electrons with each other (metallic bonds), elements on the right (non-metals) share electrons with each other (covalent bonds) and elements on the left tend to donate electrons to elements on the right (ionic bonds).
Consider, however, carbon (C) and chlorine (Cl). Chlorine is clearly to the right. Carbon is however fairly central. Electrons in a bond between these two elements are shared, but they are not shared equally. The shared electrons (one from Cl, one from C) would spend more of their time under the influence of chlorine, being farther right, but are not completely lost to carbon as they would be to sodium. (Or to caesium even more - can you suggest why?)
One word of warning: hydrogen behaves in all this with a divided personality. In those few cases in which it bonds with metals it tends to behave as a metal. NaH acts as an alloy. With non-metals hydrogen behaves as a non-metal. H2O (water), NH3 (ammonia) and CH4 (methane) are all covalent compounds. While it is traditionally placed in the periodic table above lithium, there are good reasons to put it above fluorine instead (or as well). As with all generalisations, there are exceptions. Notice HF was not in the list of covalent compounds. Fluorine and the other halogens are strong enough at grabbing electrons that even with hydrogen donating is the better description for the valence electrons.
Next Page: Comparing Bond Types
Related Topic: Two Kinds of Covalent Compounds