Lecture 34 Formulas for Ionic Compounds Ozgur Unal 1

 What do these formulas tell you about the ions in the compound?  NaClMgCl2Ca3(PO4)2 2  In this lesson we will find out how to determine the formulas for ionic compounds.  The chemical formula of an ionic compound, also called a formula unit, represents the simplest ratio of the ions involved.  Example: MgCl2 contains 1 Mg ion and 2 Chloride ions. The total charge of MgCl2 is zero. 1 Mg +2 and 2 Cl - form MgCl2.

 Binary ionic compounds are composed of positively charged monoatomic ions of metal and negatively charged monoatomic ions of nonmetal.  A monoatomic ion is a one-atom ion, such as Mg +2 and Br -.  Check out Table  The charge of a monoatomic ion is known as its oxidation number.  The oxidation number of an element in an ionic compound equals the number of electrons transferred from the atom to form the ion.  Example: NaCl. Na transfers one electron to Cl. Oxidation number of Na is +1. Oxidation number of Cl is -1.

Formulas for binary ionic compounds:  In the chemical formula for any ionic compound, the symbol of the cation is always written first, followed by the symbol of the anion.  Subscripts, which are small numbers to the lower right of a symbol, represent the number of ions of each element in an ionic compound.  Ionic compounds have no charge: If you add the oxidation number of each ion multiplied by the number of these ions in a formula unit, the total must be zero. 4  Example: X +a reacts with Y -b. The ionic compound formed by these ions is: X b Y a.

 Many ionic compounds contain polyatomic ions, which are made up of more than one atom.  Examples: OH - H2PO4 - S2O3 -2 O2 -2  A polyatomic ion acts as an individual ion in a compound and that its charge applies to the entire group of atoms.  Thus, the formula for a polyatomic compound follows the same rules used for a binary compound. 5  Because a polyatomic ion exists as a unit, never change subscripts of the atoms within the ion.  If more than one polyatomic ion is needed, place parantheses around the ion and write the appropriate subscript outside the parantheses.

 Example: The compound formed by NH4 + and O -2 is (NH4)2O. 6  Example: A compound formed by calcium ions and phosphate ions is often used in fertilizers. Write the compound’s formula.  Example: Write the formula of the ionic compound formed by sodium and nitrate.

Lecture 35 Names for Ions and Ionic Compounds Ozgur Unal 7

8  NaClMgCl2  Try to name these ionic compounds  KH2PO4Na2S2O3(NH4)2O  Try to name these ionic compounds.  It is hard to call the ionic compounds using the names of the atoms in the compound.  In this lesson we will learn how to name ionic compounds.

 An oxyanion is a polyatomic ion composed of an element, usually nonmetal, bonded to one or more oxygen atoms.  Example: SO3 -2, NO2 - etc.  There can be more than one oxyanion for some nonmetals.  Check out Table 7.10 for N and S, and Table 7.11 for Cl 9  Example:  NO3 -  Nitrate ClO4 -  Perchlorate  NO2 -  Nitrite ClO3 -  Chlorate  SO4 -2  Sulfate ClO2 -  Chlorite  SO3 -2  Sulfite ClO -  Hypochlorite  BrO3 -  Bromate  IO4 -  Periodate IO3 -  Iodate

Follow the five rules:  1- Name the cation followed by the anion. Remember that the cation is always written first in the formula.  2- For monoatomic cations, use the element name.  3- For monoatomic anions, use the root of the element name plus the suffix –ide.  4- To distinguish between multiple oxidation numbers of the same element, the name of the chemical formula must indicate the oxidation number of the cation. Oxidation number is written as a Roman numeral in parantheses after the name of the cation.  5- When the compound contains a polyatomic ion, simply name the cation followed by the name of the polyatomic ion. 10

 Examples:  NaCl  Sodium Chloride  MgCl2  Magnesium Chloride  FeO  Iron (II) oxide  Fe2O3  Iron (III) oxide  (NH4)2S  Ammonium Sulfide  NaOH  Sodium Hydroxide  Cu(NO3)2  Copper (II) Nitrate  KClO4  Potassium Perchlorate 11