Writing Ionic Formulas

Ionic Compounds

 An ionic compound is always made up of a metal and a nonmetal.

 Ionic compounds can be binary (made of just two elements), or contain one or more polyatomic ions as part of it (many different elements).

 To determine the formula for an ionic compound, we will use the criss-cross method.

The Criss-Cross Method

 The

criss-cross method

is an easy way to determine the formula of any ionic compound.

 To write an ionic formula, you need the

charge

each ion.

of  The charge is found by looking at your periodic table for most ions.

Steps

1.

How to Criss-Cross

(in order)

Example:

calcium + nitrogen Write the chemical 1.

Ca N symbols. Always write the cation + (metal) first and anion – 2.

Ca 2+ N 3 (nonmetal) second.

2.

Write the charge over the symbols.

3.

Ca 3 N 2 3.

4.

Criss-cross and drop numbers (not + or -).

Reduce – if necessary – the same numbers or a 2 & 4 together.

Steps

1.

How to Criss-Cross

(in order)

Example:

aluminum + oxygen Write the chemical 1.

Al O symbols. Always write the cation + (metal) first and anion – 2.

Al 3+ O 2 (nonmetal) second.

2.

Write the charge over the symbols.

3.

Al 2 O 3 3.

4.

Criss-cross and drop numbers (not + or -).

Reduce – if necessary – the same numbers or a 2 & 4 together.

Steps

1.

How to Criss-Cross

(in order)

Example:

sodium + sulfur Write the chemical 1.

Na S symbols. Always write the cation + (metal) first and anion – 2.

Na 1+ S 2 (nonmetal) second.

2.

Write the charge over the symbols.

3.

Na 2 S 1’s are understood, you do not need to write them!

3.

Criss-cross and drop numbers (not + or -).

4.

Reduce – if necessary – the same numbers or a 2 & 4 together.

Steps

1.

How to Criss-Cross

(in order)

Example:

aluminum + nitrogen Write the chemical 1.

Al N symbols. Always write the cation + (metal) first and anion – 2.

Al 3+ N 3 (nonmetal) second.

2.

Write the charge over the symbols.

3.

Al 3 N 3 4.

AlN 3.

Criss-cross and drop numbers (not + or -).

4.

Reduce – if necessary – the same numbers or a 2 & 4 together.

Try These Formulas

Use the criss-cross method to find the formulas for the following.

Ions in Compound

         Beryllium + bromine Lithium + oxygen Potassium + nitrogen Calcium + sulfur Magnesium + nitrogen Aluminum + phosphorus Lithium + chlorine Magnesium + oxygen Calcium + iodine

Chemical Formula

         BeBr 2 Li 2 O K 3 N CaS Mg 3 N 2 AlP LiCl MgO CaI 2



Formulas With Transition Metals

Transition metals (groups 3-12) can often form more than one type of cation.

 Lead (Pb) and Tin (Sn) can also form more than one type of cation.

 The charge for these metals is always positive and can be found in the name of the compound that contains them (the Roman numeral indicates the charge).

 Examples:  

Iron (II)

– the iron (Fe) used here has a charge of 2+.

Lead (IV)

– the Lead (Pb) used here has a charge of 4+.

1.

2.

3.

4.

5.

6.

Formulas With Transition Metals

Write the formulas for each of the following ionic compounds that feature one of the transition metals.

Name of Compound Chemical Formula

Iron (III) and oxgen Tin (II) and sulfur Lead (II) and chlorine Iron (II) and nitrogen Lead (IV) and oxygen Silver (I) and phosphorus 1.

2.

3.

4.

5.

6.

Fe 2 O 3 SnS PbCl 2 Fe 3 N 2 PbO 2 Ag 3 P



Polyatomic Ionic Compounds

Polyatomic ions

are clusters of atoms that stay together as one unit and carry an overall charge. Most polyatomic ions are negatively charged.

Parentheses

 You may need more than one polyatomic ion in your formula…This means that you will have to use parentheses. Use parentheses whenever you need more than one polyatomic ion in the formula.

 Example: Iron (III) nitrate…  Start with symbols and combining charges…  Fe 3+ NO 3 1 After the criss-cross , you may get one of two possibilities…  FeNO 33

OR

Fe(NO 3 ) 3 The second option is right because it says you need two nitrate clusters to go with every iron atom. The first option, which did not use brackets, reads as one iron atom with one nitrogen atom and thirty-three oxygen atoms.



Fe(NO 3 ) 3

is the correct formula! Use parentheses!

Polyatomic Ionic Compounds

Write the formulas for each of the following polyatomic ionic compounds. Use parentheses when necessary.

Name of Compound

          Sodium nitrate Calcium chlorate Lithium phosphate Aluminum sulfate Tin (IV) carbonate Iron (II) hydroxide Magnesium carbonate Lead (II) bicarbonate Potassium chlorate Ammonium phosphate

Chemical Formula

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

NaNO 3 Ca(ClO 3 ) 2 Li 3 PO 4 Al 2 (SO 4 ) 3 Sn(CO 3 ) 2 Fe(OH) 2 MgCO 3 Pb(HCO 3 ) 2 KClO 3 (NH 4 ) 3 PO 4