How do you write the formula for calcium fluoride?

To write the formula for calcium fluoride, follow these steps:

1. **Identify the Ions**: Calcium (Ca) and fluorine (F) are the two elements involved. Calcium is a metal and forms a cation (positively charged ion), while fluorine is a non-metal and forms an anion (negatively charged ion).

2. **Determine Charges**:
- Calcium typically has a charge of \( +2 \) because it loses two electrons to achieve a stable electron configuration (it has 2 electrons in its outer shell).
- Fluorine typically has a charge of \( -1 \) because it gains one electron to achieve a stable electron configuration (it has 7 electrons in its outer shell and needs one more to complete it).

3. **Balancing the Charges**: To create a neutral compound, the total positive charge must balance the total negative charge. Since calcium has a \( +2 \) charge and fluorine has a \( -1 \) charge, you will need two fluoride ions to balance the charge of one calcium ion:
- \( \text{Ca}^{2+} \)
- \( \text{F}^{-} \) (two of these, since -1 charge x 2 = -2 charge)

4. **Write the Formula**: Combine the ions, placing the cation first and the anion second. The number of each ion is indicated by subscripts:
- The formula for calcium fluoride is written as \( \text{CaF}_2 \).

### Final Answer
The chemical formula for calcium fluoride is **CaF₂**.