Which of the following is closest to the atomic mass of argon (Ar)?

• A. 22.99 u
• B. 35.45 u
• C. 39.95 u
• D. 18.00 u

Answer: (C)

Verified step by step guidance

1. Understand that the atomic mass of an element is the weighted average mass of all its naturally occurring isotopes, measured in atomic mass units (u).
2. Recall or look up the atomic masses of the given options and recognize that argon (Ar) is a noble gas with an atomic mass close to 40 u.
3. Compare the given options: 22.99 u (close to sodium), 35.45 u (close to chlorine), 39.95 u, and 18.00 u (close to oxygen).
4. Identify that the atomic mass of argon is closest to 39.95 u based on its position in the periodic table and known atomic masses.
5. Confirm that 39.95 u is the correct choice as it matches the accepted atomic mass of argon.