ionic formulas
Chemical formulas for ionic compounds are called ionic formulas. A proper ionic formula has a cation and an anion in it; an ionic compound is never formed between two cations only or two anions only. The key to writing proper ionic formulas is simple: the total positive charge must balance the total negative charge. Because the charges on the ions are characteristic, sometimes we have to have more than one of a cation or an anion to balance the overall positive and negative charges. It is conventional to use the lowest ratio of ions that are needed to balance the charges. For example, consider the ionic compound between Na+ and Cl. Each ion has a single charge, one positive and one negative, so we need only one ion of each to balance the overall charge. When writing the ionic formula, we follow two additional conventions: 1 write the formula for the cation first and the formula for the anion next, but 2 do not write the charges on the ions. Thus, for the compound between Na+ and Cl, we have the ionic formula NaCl Figure 3.5 "NaCl = Table Salt" . The formula Na2Cl2 also has balanced charges, but the convention is to use the lowest ratio of ions, which would be one of each. Remember from our conventions for writing formulas that we dont write a 1 subscript if there is only one atom of a particular element present. For the ionic compound between magnesium cations Mg2+ and oxide anions O2 , again we need only one of each ion to balance the charges. By convention, the formula is MgO. For the ionic compound between Mg2+ ions and Cl ions, we now consider the fact that the charges have different magnitudes, 2+ on the magnesium ion and 1 on the chloride ion. To balance the charges with the lowest number of ions possible, we need to have two chloride ions Saylor URL: http://www. saylor. org/books.