Which element has 18 protons in its nucleus?

• A. Calcium (Ca)
• B. Potassium (K)
• C. Chlorine (Cl)
• D. Argon (Ar)

Answer: (D)

Verified step by step guidance

1. Recall that the number of protons in the nucleus of an atom is called the atomic number, which uniquely identifies an element.
2. Look up the atomic numbers of the given elements: Calcium (Ca), Potassium (K), Chlorine (Cl), and Argon (Ar).
3. Compare each element's atomic number to 18, since the problem asks for the element with 18 protons.
4. Identify the element whose atomic number is exactly 18, which means it has 18 protons in its nucleus.
5. Confirm that this element is Argon (Ar), as it has an atomic number of 18.