Chapter 7- Ionic Compounds and Metals
Section 3- Naming and Formulas for Ionic Compounds
Section 3 Learning Objectives:
1. Relate a formula unit of an ionic compound to its composition.
2. Write formulas for ionic compounds and oxyanions.
3. Apply naming conventions to ionic compounds and oxyanions.
State of Ohio and WCS Joint “I CAN STATEMENTS”:
_____ Apply and use the rules for writing and naming both binary and polyatomic ionic compounds.
_____ Recognize that subscripts are used to indicate the number of ions present in a given formula.
_____ Recognize that the sum of the ionic charges must equal zero for any stable compound.
_____ Reduce subscripts to the simplest ratio in ionic compounds.
_____ Use the stock system to properly name compounds
VOCABULARY
Formula unit, monatomic ion, oxidation number, polyatomic ion, oxyanions
Formulas for Ionic Compounds
- Smallest ratio of ions in an ionic compound is called a formula unit. Cations are written first, anions second.
Ex. NaCl has a formula unit of one Na + ion to one Cl – ion.
Ex. MgCl2 has a formula unit of one Mg 2+ ion to two Cl – ions.
Monatomic ions
- A monatomic ion is an ion made from one atom.
Ex. Na +, Mg 2+ and Al 3+ are all types of monatomic cations.
Ex. Cl-, O 2-, and N 3- are all types of monatomic anions.
- All binary ionic compounds contain monatomic positive metal ions and monatomic negative nonmetal anions.
Oxidation numbers
- The charge of a monatomic ion is also called its oxidation number.
- It tells how many electrons are transferred to or from an ion.
- Some elements have more than one oxidation number, indicating they form multiple ions.
Formulas for binary ionic compounds
- Ionic compound formulas show the ratio of ions to get a zero net charge.
- Determining formula if given charges of ions:
- Cross the charge value down to be the superscript of the other ion. Remove the + or - value.
Ex. K + O 2-
Ex. Al 3+ S 2-
K2O
Al2S3
Ex. potassium and iodide
KI
Ex. magnesium and chloride
MgCl2
Ex. cesium and nitride
Cs3N2
Formulas for polyatomic ionic compounds
- Many ionic compounds contain polyatomic ions, which are ions made from more than one ion (pg 221, 970).
- Formulas containing polyatomic ions are written the same way as binary ionic compounds, except a
parenthesis is placed around ones that already have a subscript before the charge is brought down.
Ex. NH4 + and O 2-
Ex. Ca 2- and PO4 3-
Ex. sodium and nitrate
Ex. calcium and chlorate
Ex. aluminum and carbonate
(NH4)2O
Ca3(PO4)2
NaNO3
Ca(ClO3)2
Al2(CO3)3
Formulas for Binary Ionic Compounds with Transition Metals
-Formulas for binary ionic compounds containing transition metals are written the exact same way as normal
binary ionic compounds.
- Many transition metals contain ions with multiple oxidation numbers. The oxidation number is written with a
roman numeral.
Ex. Fe 2+ and Cl -
Ex. Mn 2+ and O 2-
Ex. chromium (II) and fluoride
FeCl2
MnO
CrF2
Ex. copper (I) sulfide
Cu2S
Ex. cobalt (III) oxide
Co2O3
Names for Ions and Ionic Compounds
Naming ionic compounds
MONATOMIC IONS
1. Cation is written/named first. Use metal name.
2. Anion is written/named second. Anions- keep the root of the element and add the suffix –ide.
Ex. Cs Br is called cesium bromide
Ex. Na Br is called sodium bromide.
3. Many transition metals have multiple oxidation states and a roman numeral must be added to identify the
correct oxidation state of the cation (refer to the chart).
Ex.
Fe 2+ and O 2- is called iron (II) oxide
Ex. Cu + and S 2- is called copper (I) sulfide
Fe 3+ and O 2- is called iron (III) oxide
Ex. Cu 2+ and S 2- is called copper (II) sulfide
POLYATOMIC IONS
4. If the compound contains polyatomic ions, simply keep their names and follow rules 1-3 for the other ion. If
the compound contains two polyatomic ions, record both names.
Ex.
KOH is called potassium hydroxide
Ex. Na2CO3 is called sodium carbonate
Ex.
NH4Cl is called ammonium chloride
Ex. (NH4)3PO4 is called ammonium phosphate
Naming an oxyanions
- Oxyanions are polyatomic ions that contain (usually) a nonmetal ion bonded to one or more oxygen ions.
- Any oxyanion ending in the suffix -ate is called the standard form.
Ex. ClO3 - is called chlorate
- If an oxygen is removed or missing, the ending is changed to – ite.
Ex. ClO2 - is called chlorite
- If two oxygen atoms are removed, the – ite remains, but the prefix hypo is added.
Ex. ClO - is called hypochlorite
- If an extra oxygen atom is added to the standard form, only the prefix per is added to the standard form.
Ex. ClO4 - is called perchlorate