Which element has an atomic weight approximately 36?

• A. B C N O F (10, 12, 14, 16, 18)
• B. Cl (36)
• C. Na (23)
• D. H (1)

Answer: (B)

Atomic weight is the weighted average mass of an element’s atoms, taking into account the natural isotopic mix. Chlorine has two common isotopes, 35Cl and 37Cl, in roughly 75:25 abundance, which gives a weighted average around 35.5 amu, often rounded to 36. That makes chlorine the element with a mass near 36. The other options are much lighter: hydrogen is about 1 amu, sodium about 23 amu, and the group listed for the first set contains masses in the 10–18 range, far from 36.