Which element forms an ion that has 17 protons and 18 electrons?

• A. Sulfur (forming $S^{-}$)
• B. Chlorine (forming $C^l$)
• C. Potassium (forming $K^{-}$)
• D. Argon (forming $A^r$)

Answer: (B)

Verified step by step guidance

1. Recall that the number of protons in an atom defines the element. Here, the ion has 17 protons, so identify the element with atomic number 17 on the periodic table.
2. Determine the element with atomic number 17, which is Chlorine (Cl). This means the ion is derived from chlorine.
3. Understand that the ion has 18 electrons, which is one more electron than the number of protons (17). This indicates the ion has a charge of -1, since electrons are negatively charged.
4. Recognize that when chlorine gains one electron, it forms the chloride ion, written as Cl⁻, which has 17 protons and 18 electrons.
5. Confirm that the other options do not match these criteria: sulfur has 16 protons, potassium has 19 protons, and argon has 18 protons, so they cannot form an ion with 17 protons.