Which element has an atomic weight of about 40?

Which element has an atomic weight of about 40?

• A. B C N O F (10, 12, 14, 16, 18)
• B. Cl (36)
• C. Na (23)
• D. Ca (40)

Answer: (D)

Atomic weight is the weighted average mass of an element’s naturally occurring atoms, reflecting the abundances of its isotopes. Calcium has a dominant isotope with mass number 40, and smaller amounts of other isotopes such as 42, 43, and 44. When these are averaged, the result is about 40. That’s why calcium sits near 40 on the periodic table. The other elements listed have atomic weights well below 40—for example, boron around 11, carbon about 12, nitrogen about 14, oxygen about 16, fluorine about 19, chlorine around 35.5, and sodium about 23—so they don’t match the “about 40” value.

Atomic weight is the weighted average mass of an element’s naturally occurring atoms, reflecting the abundances of its isotopes. Calcium has a dominant isotope with mass number 40, and smaller amounts of other isotopes such as 42, 43, and 44. When these are averaged, the result is about 40. That’s why calcium sits near 40 on the periodic table. The other elements listed have atomic weights well below 40—for example, boron around 11, carbon about 12, nitrogen about 14, oxygen about 16, fluorine about 19, chlorine around 35.5, and sodium about 23—so they don’t match the “about 40” value.