Steps:
1. Identify the ions: Determine the cation (positive ion) and anion (negative ion) that will form the compound.
2. Determine the charges: Find the charges of each ion. You can often look these up on a periodic table or in a list of common ions.
3. Balance the charges: The total positive charge must equal the total negative charge.
* If the charges are equal, the formula is simply the cation followed by the anion.
* If the charges are not equal, you need to find the least common multiple (LCM) of the charges. Then, use subscripts to indicate the number of each ion needed to achieve that LCM.
4. Write the formula: Write the symbols of the ions with their respective subscripts.
Example:
Let's find the formula for sodium chloride (table salt).
1. Ions: Sodium (Na) is the cation, and chloride (Cl) is the anion.
2. Charges: Sodium has a +1 charge (Na⁺), and chloride has a -1 charge (Cl⁻).
3. Balance: The charges are already equal (+1 and -1).
4. Formula: NaCl
Another Example:
Let's find the formula for calcium oxide.
1. Ions: Calcium (Ca) is the cation, and oxygen (O) is the anion.
2. Charges: Calcium has a +2 charge (Ca²⁺), and oxygen has a -2 charge (O²⁻).
3. Balance: The charges are already equal (+2 and -2).
4. Formula: CaO
Example with unequal charges:
Let's find the formula for aluminum oxide.
1. Ions: Aluminum (Al) is the cation, and oxygen (O) is the anion.
2. Charges: Aluminum has a +3 charge (Al³⁺), and oxygen has a -2 charge (O²⁻).
3. Balance: The LCM of 3 and 2 is 6. We need two aluminum ions (2 x +3 = +6) and three oxygen ions (3 x -2 = -6).
4. Formula: Al₂O₃
Key Points:
* Ionic compounds are neutral.
* The subscripts in an ionic formula represent the number of each ion present, not the number of atoms within the ion.
* Always simplify the subscripts to their lowest whole-number ratio.
