Ionic Bonding
3. Ionic Compound Formulas
Learning outcomes
- I can write chemical formulas for ionic compounds.
- I can determine ion ratios based on charge balance.
- I can name common ionic compounds.
- I can identify the ions present in a compound.
- I can apply charge balance to construct formulas.
Introduction
Chemists use chemical formulas to show which elements are present in a compound and the ratio in which their atoms or ions combine.
For ionic compounds, the formula is not chosen randomly. It is determined by one simple but important rule:
The total positive charge must equal the total negative charge.
This rule, known as charge balance, allows us to predict the correct formula for almost any ionic compound.
Why Do Ionic Compounds Need Balanced Charges?
Every ionic compound must be electrically neutral.
This means:
Total positive charge = Total negative charge
For example:
Na⁺ + Cl⁻
Total charge:
+1 + (−1) = 0
Formula:
NaCl
Only one sodium ion and one chloride ion are needed.
Finding the Correct Ion Ratio
Sometimes one positive ion is not enough to balance one negative ion.
Instead, we need the correct ratio of ions.
Example 1
Magnesium forms:
Mg²⁺
Chlorine forms:
Cl⁻
One Mg²⁺ has a charge of +2.
Each Cl⁻ has a charge of −1.
Two chloride ions are needed.
Formula:
MgCl₂
Example 2
Calcium oxide
Calcium:
Ca²⁺
Oxygen:
O²⁻
Charges:
+2 and −2
One of each balances perfectly.
Formula:
CaO
Example 3
Aluminium oxide
Aluminium:
Al³⁺
Oxygen:
O²⁻
The smallest combination that balances the charges is:
2 Al³⁺ = +6
3 O²⁻ = −6
Formula:
Al₂O₃
Using Charge Balance
A simple method for writing ionic formulas is:
Step 1
Write the ion symbols and charges.
Example:
Ca²⁺
Cl⁻
Step 2
Determine how many of each ion are needed so the total charge equals zero.
Step 3
Write the chemical formula.
Result:
CaCl₂
Remember:
The charges are not written in the final formula.
Common Ionic Charges
The periodic table helps predict the charges of many common ions.
| Group | Common Ion |
|---|---|
| Group 1 | +1 |
| Group 2 | +2 |
| Group 13 | +3 |
| Group 15 | –3 |
| Group 16 | –2 |
| Group 17 | –1 |
Some common ions are:
| Positive Ions | Negative Ions |
|---|---|
| Li⁺ | F⁻ |
| Na⁺ | Cl⁻ |
| K⁺ | Br⁻ |
| Mg²⁺ | O²⁻ |
| Ca²⁺ | S²⁻ |
| Al³⁺ | N³⁻ |
Naming Ionic Compounds
Naming ionic compounds is straightforward.
Rule
- Name the positive ion first.
- Name the negative ion second.
- Change the ending of the non-metal to -ide.
Examples:
| Formula | Name |
|---|---|
| NaCl | Sodium chloride |
| MgO | Magnesium oxide |
| CaBr₂ | Calcium bromide |
| LiF | Lithium fluoride |
| K₂S | Potassium sulfide |
| AlN | Aluminium nitride |
Identifying the Ions
You should also be able to determine which ions make up a compound.
Example:
NaCl
Contains:
- Na⁺
- Cl⁻
Example:
MgCl₂
Contains:
- Mg²⁺
- 2 Cl⁻
Example:
Al₂O₃
Contains:
- 2 Al³⁺
- 3 O²⁻
The Criss-Cross Method
A useful shortcut is the criss-cross method.
Take the charge numbers (not the signs) and use them as subscripts.
Example:
Al³⁺
O²⁻
Cross the numbers:
Al₂O₃
However, always simplify the ratio if possible.
Example:
Ca²⁺
O²⁻
Criss-cross gives:
Ca₂O₂
Simplify:
CaO
The formula should always show the smallest whole-number ratio of ions.
Real-World Applications
Writing ionic formulas is important in:
- Medicine
- Agriculture
- Chemical manufacturing
- Water treatment
- Food production
- Materials science
- Environmental science
- Laboratory chemistry
Chemists use formulas to communicate exactly which substances are involved in chemical reactions.
Worked Examples
Example 1
Write the formula for sodium chloride.
Na⁺
Cl⁻
Charges balance 1:1.
Answer:
NaCl
Example 2
Write the formula for magnesium oxide.
Mg²⁺
O²⁻
Charges balance 1:1.
Answer:
MgO
Example 3
Write the formula for calcium chloride.
Ca²⁺
Cl⁻
Two chloride ions are needed.
Answer:
CaCl₂
Example 4
Write the formula for aluminium oxide.
Al³⁺
O²⁻
Balance:
2 Al³⁺
3 O²⁻
Answer:
Al₂O₃
Example 5
Name the compound:
K₂S
Contains:
K⁺
S²⁻
Answer:
Potassium sulfide
Example 6
Identify the ions in MgBr₂.
Answer:
- Mg²⁺
- 2 Br⁻
Did You Know?
The chemical formulas of ionic compounds always represent the simplest ratio of ions, not the actual number of ions in a crystal. A tiny crystal of sodium chloride contains billions of sodium and chloride ions arranged in a giant repeating lattice, yet its formula is simply NaCl because the ratio of sodium ions to chloride ions is always 1:1.
Key Terms
| Term | Definition |
|---|---|
| Chemical Formula | A combination of symbols showing the elements or ions present in a compound and their simplest ratio. |
| Charge Balance | The requirement that the total positive and negative charges in a compound are equal. |
| Subscript | A small number written after a chemical symbol showing how many atoms or ions are present. |
| Ion Ratio | The simplest whole-number ratio of positive and negative ions in an ionic compound. |
| Cation | A positively charged ion. |
| Anion | A negatively charged ion. |
| Ionic Compound | A compound made from positive and negative ions held together by ionic bonds. |
Key Takeaways
- Ionic formulas are determined by balancing positive and negative charges.
- Every ionic compound must have an overall charge of zero.
- Subscripts show the ratio of ions, not their charges.
- The formula always uses the smallest whole-number ratio of ions.
- Ionic compounds are named by writing the positive ion first and the negative ion second, with the non-metal ending changed to -ide.
- Understanding charge balance allows you to write and interpret formulas for a wide range of ionic compounds.