An ion ${\mathrm{X}}^{-}$ contains 18 electrons and 18 neutrons. What is the atomic number of the element X?

1. Given the anion ${\mathrm{X}}^{-}$ has 18 electrons and 1 negative charge.
2. The charge tells about the number of accepted electrons by the neutral atom.
3. For the ion ${\mathrm{X}}^{-}$, it has accepted 1 electron. Therefore, the number of electrons in the neutral atom must have been 17.
4. As atomic number = number of electrons in neutral atom.
5. Therefore, the atomic number of the element X is 17.