1. **State the problem:** We need to find the charge of a fluorine ion with 9 protons and 10 electrons. 2. **Formula:** The total charge $Q$ of an ion is given by: $$Q = (\text{charge per electron})(\text{number of electrons}) + (\text{charge per proton})(\text{number of protons})$$ 3. **Substitute values:** Each electron has a charge of $-1$, and there are 10 electrons. Each proton has a charge of $+1$, and there are 9 protons. So, $$Q = (-1)(10) + (+1)(9)$$ 4. **Calculate:** $$Q = -10 + 9$$ 5. **Simplify:** $$Q = -1$$ 6. **Interpretation:** The fluorine ion has a net charge of $-1$, meaning it has one more electron than protons, making it negatively charged.