The mass numbers of the two naturally occurring isotopes of antimony are 121 and 123. The atomic mass of antimony on the periodic table is 121.76. Explain why this atomic mass is not a whole number.

• A. The atomic mass of antimony on the periodic table is not a whole number because it is the weighted average of the mass numbers of its naturally occurring isotopes.
• B. The atomic mass of antimony on the periodic table is not a whole number because it is the sum of the actual atomic masses of its naturally occurring isotopes.
• C. The atomic mass of antimony on the periodic table is not a whole number because it is the average of the mass numbers of its two heaviest isotopes.
• D. The atomic mass of antimony on the periodic table is not a whole number because it is the sum of the mass numbers of its two heaviest isotopes.

Answer: (A)