Which isotope has 18 protons and 22 neutrons?

• A. Sulfur-40 (S-40)
• B. Potassium-40 (K-40)
• C. Calcium-40 (Ca-40)
• D. Argon-40 (Ar-40)

Answer: (D)

Verified step by step guidance

1. Identify the element by the number of protons, since the number of protons defines the element's atomic number. Here, the element has 18 protons.
2. Recall the periodic table to find which element has an atomic number of 18. This element is Argon (Ar).
3. Calculate the mass number of the isotope by adding the number of protons and neutrons: \(\text{mass number} = \text{protons} + \text{neutrons} = 18 + 22\).
4. Write the isotope notation using the element symbol and the mass number, which will be \(^{40}\text{Ar}\) (Argon-40).
5. Compare this with the given options to confirm that Argon-40 is the correct isotope with 18 protons and 22 neutrons.