Ions and Ionic Compounds

NaCl is an ionic compound composed of the Na⁺ and Cl^- ions. The reason they have charges is because they exchange electrons. In the picture below, the X's and dots represent the electrons in a simple chemical reaction.





We see that the sodium atom has lost one negatively charged electron. That is why it has a 1+ charge. The chlorine atom has taken one electron from the sodium, so it has a 1- charge. We say that sodium has an oxidation number of +1 and chlorine has an oxidation number of -1.

Note that charges on ions are always written as "1+", while oxidation numbers are written as "+1". While moving the "+" or "-" sign may seem "nitpicky," there is a good reason for it. Oxidation numbers can be used for chemicals that are not ionic and do not have any charges. We move the sign so oxidation numbers are not confused with ionic charges. We will take a look at these compounds on the next page.