Here's how to determine the correct symbol for a sulfur ion with 18 electrons:

1. Sulfur's Atomic Number: Sulfur (S) has an atomic number of 16, meaning a neutral sulfur atom has 16 protons and 16 electrons.

2. Calculating the Charge: To have 18 electrons, the sulfur atom must have gained 2 extra electrons. This gives it a charge of -2.

3. The Symbol: The correct symbol for a sulfur ion with 18 electrons is S²⁻.