Which element has an atomic weight of about 23?

Which element has an atomic weight of about 23?

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

Answer: (C)

Atomic weight is the average mass of an element’s atoms, expressed in atomic mass units, reflecting the natural mix of isotopes. Sodium has a mass about 23 amu because its most abundant isotope is Na-23, making its average atomic weight close to 23. Therefore, the element with an atomic weight about 23 is sodium. The others listed are notably lighter or heavier—hydrogen around 1, carbon about 12, nitrogen about 14, oxygen about 16, fluorine about 19, and chlorine around 35.5—so they do not fit the 23-amu mark.

Atomic weight is the average mass of an element’s atoms, expressed in atomic mass units, reflecting the natural mix of isotopes. Sodium has a mass about 23 amu because its most abundant isotope is Na-23, making its average atomic weight close to 23. Therefore, the element with an atomic weight about 23 is sodium. The others listed are notably lighter or heavier—hydrogen around 1, carbon about 12, nitrogen about 14, oxygen about 16, fluorine about 19, and chlorine around 35.5—so they do not fit the 23-amu mark.