If the mass of Br-81 is 80.9163 amu, which of the following is closest to the mass of Br-79?

• A. 78.9183 amu
• B. 79.9040 amu
• C. 81.0000 amu
• D. 80.9163 amu

Answer: (A)

Verified step by step guidance

1. Understand that bromine has two main isotopes: Br-79 and Br-81, where the numbers represent the mass numbers (total protons and neutrons). The problem gives the mass of Br-81 and asks for the mass of Br-79.
2. Recall that the mass number is approximately the sum of protons and neutrons, but the actual atomic mass (in amu) is slightly different due to nuclear binding energy and mass defects.
3. Since the mass of Br-81 is given as 80.9163 amu, and Br-79 is lighter by 2 mass units (81 - 79 = 2), expect the mass of Br-79 to be roughly 2 amu less than Br-81, but not exactly 2 amu less due to the mass defect.
4. Compare the given options to the expected value: subtract approximately 2 amu from 80.9163 amu to estimate the mass of Br-79, then identify which option is closest to this estimate.
5. Select the option closest to this estimated mass, which should be near 78.9 amu, reflecting the actual atomic mass of Br-79.