3.2 NAMES AND FORMULAS OF IONIC COMPOUNDS BC Science 9: p
NAMING IONIC COMPOUNDS The chemical name indicates the elements present in the compound. Chemical names for the ionic compounds are given according to rules. The positive ion is always the first part of the name. The negative ion is always the second part of the name. The non-metal ion’s name ends with the suffix “ide.”
NAMING IONIC COMPOUNDS
IONIC CHEMICAL FORMULAS In an ionic compound, the positive charges balance (ie. cancel out) the negative charges. This balance of charge is used to determine the smallest whole number ratio of positive to negative ions.
IONIC CHEMICAL FORMULAS
MULTIVALENT METAL COMPOUNDS Many metals are multivalent (ie. two or more possible ion charges). Eg. Iron atoms form both Fe +2 and Fe +3. To distinguish between different ions for multivalent metals, Roman numerals are added to their name. Eg. Fe +2 is Iron (II), and Fe +3 is Iron (III).
MULTIVALENT METAL COMPOUNDS
Writing ionic compound formulas with multivalent ions follows the same rules as regular ionic compounds.
MULTIVALENT COMPOUND NAMES Steps to writing multivalent compound names are as follows: Identify the metal and verify it forms more than one ion. Determine the ratio of ions. Eg. Fe 2 O 3 means 2 iron ions for every 3 oxygen ions. Note the charge on the negative ion: Oxygen is O -2. The positive and negative charges must be equal but opposite, so 2 Fe +3 ions are needed. Write the name of the compound: Iron (III) Oxide.
POLYATOMIC ION COMPOUNDS Steps to writing names and formulas involving polyatomic ions are similar to other ionic compounds.
POLYATOMIC ION COMPOUNDS All polyatomic ions have negative charges with the exception of Ammonium (NH 4 + ), and will be written second in the formula. Brackets must be used in the formula if two or more polyatomic ions are present in the compound. A subscript number outside the brackets is used. Compounds containing a polyatomic ion and a multivalent metal must still indicate the metal ion by using a Roman numeral in the name.