Writing Formulas forIonic Compounds

Anatomy of aChemical Formula

Chemical formulas express which elements have bonded to form a compound. The subscripts express the elements' numerical relationships to one another.

Formula Unit foran Ionic Compound

Since ionic compounds do not exist as single units, the formula of an ionic compounds represents the simplest ratio of ions. It is called a FORMULA UNIT.

Example: MgCl2 is a formula unit of magnesium chloride.

Cations are written first in the formula unit; anions second.

Formula Unit foran Ionic Compound

The charges of the ions that make up an ionic compound must cancel out - they must add up to zero!

Example: MgCl2 is made up of one Mg+2 ion and 2 Cl- ions. The charges cancel (add uo to zero): +2 + 2(-1) = 0

An easy way to write a chemical formula in which the charges of

the cations and anions cancel each other out is to use the criss-

cross method.

Criss-cross method1. Determine the ions (their symbols and their charges) that are present in the compound2. Take the NUMBER ONLY of the charge for the

cation and write it as a subscript after the anion3. Take the NUMBER ONLY of the charge for the

anion and write it as a subscript after the cation

Example: Magnesium chloride

Mg+2 Cl-

1 becomes the 2 becomes the

subscript of Mg subscript of Cl

MgCl2

ExceptionsCa2+ and O2- would form Ca2O2 if you used

the criss-cross methodHowever, in reality, Ca2+ and O2- form CaO.For ionic compounds, you must simplify

(reduce) the subscripts when necessary.

Practice ProblemsWrite the formula for the compounds listed

below. potassium oxide lithium nitride calcium phosphide magnesium sulfide aluminum oxide aluminum iodide

Remember: the name of the anion is the root of the element’s name with the suffix –ide!

Writing Formulas for Compoundswith Transition Metals

The charge of transition metal ions and Group 4A metal ions can vary. Example: Iron can form a +2 or +3 ion (Fe+2 or Fe+3); Lead can form a +2 or +4 ion (Pb+2 or Pb+4)

In most cases, a roman numeral written in parentheses after the metal in the compound’s name will tell you the charge of the metal.

When this information is NOT given to you, it means the transition metal has only one possible charge. To find out what that is, you must look on your “ion sheet”.

Writing Formulas for Compoundswith Transition Metals

Examples iron (II) oxide contains Fe+2 and the

compound’s formula is FeO iron (III) oxide contains Fe+3 and the

compound’s formula is Fe2O3

Zinc oxide contains Zn+2 and the compound’s formula is ZnO.

Practice Problems

Write the formulas for the following compounds: cobalt (III) sulfide mercury (I) chloride chromium (VI) oxide tin (IV) nitride cobalt (II) sulfide

Writing the Formula Unitfor an Ionic Compound

Binary ionic compounds are made up of 2 kinds of elements.

Polyatomic ionic compounds are made up of more than 2 kinds of elements.

(Ex. NaOH) One or both of the ions in such a compound must be a

polyatomic ion (an ion made up of more than 1 kind of atom). In our example, Na+

is the cation and OH- is the polyatomic anion. OH- contains O and H!

Polyatomic IonsThe charge of the polyatomic ion

applies to the entire ion (all of the atoms as a group)

A polyatomic ion acts as if it were an ion with one kind of atom. The different atoms act together, as a group.

Polyatomic ionsExample: NH4

+ is a polyatomic cation made up of one N atom and 4 H atoms bonded together. The overall charge on this bonded group is +1.

Example: OH- is a polyatomic anion made up of 1 O atom and 1 H atom bonded together. The bonded atoms have an overall charge of -1.

Writing Formulas for Compounds with Polyatomic Ions

Determine the ions (and their charges) that are present in the compound. If one is a polyatomic ion, use your “ion sheet” to find the formula of the ion and its charge.

Writing Formulas for Compounds with Polyatomic Ions

Follow the criss-cross rules with 1 exception: If more than one polyatomic ion is

required in the compound’s formula, the formula of the polyatomic ion MUST BE set off by parentheses.

The subscript will be written to the right of the parentheses to tell how many units of the polyatomic ions are present.

Writing Formulas for Ionic Compounds

Example: magnesium nitrate Mg2+ is the cation; NO3

- is the anion.

NEVER EVER

change the subscripts of the atoms within a polyatomic ion.

Magnesium chlorate

Mg2+is the cation; ClO3- is the anion; Mg(ClO3)2

is the formula of the compound.

NOT Mg2ClO3

MgCl2O6

Mg3ClO2

Mg2ClO3

Practice ProblemsWrite the formulas for the following

compounds: calcium hydroxide tin (IV) sulfate aluminum carbonate nickel (II) nitrate ammonium phosphate

Naming Ionic Compounds

Rules for Naming Ionic Compounds

Binary ionic compounds Name the cation first and the anion second. Monatomic cations use the element name. Monatomic anions take their name from the

root of the element name plus the suffix –ide.

Li3P

lithium phosphide

Rules for Naming Ionic Compounds

Ionic compounds with transition metals The charge of the transition metal ion must be

determined. You must work back from the formula to

determine the charge of the transition metal ion.

Once it is determined, use parentheses after the metal ion name to indicate this charge.

Co2S3

Co3+ S2-

The name is cobalt (III) sulfide

The subscript on

the S indicates

the charge of Co

is 3+

The subscript on the Co

indicates the charge of S

is 2-

Rules for Naming Ionic Compounds

If the compound contains any polyatomic ion, name the ion to name the compound.

NaHCO3

Sodium hydrogen carbonate

or

sodium bicarbonate

Practice Problems

1. CaSO4

2. CuO

3. AlBr3

4. Mg(OH)2

5. FePO4

6. NH4NO3