Question

bromine has two naturally occurring isotopes: br - 79 (abundance = 50.69%, mass = 78.918 u) and br - 81 (abundance = 49.31%, mass = 80.917 u). would you predict the atomic mass of bromine to be closer to 78.92 u, 80.92 u, or a value close to halfway in between these values? explain your reasoning. to check your answer, calculate the average atomic mass of br using the isotopic compositions given.

Explanation:

Step1: Predict the atomic mass

Since the abundance of Br - 79 (50.69%) and Br - 81 (49.31%) are very close, the atomic mass of bromine should be close to halfway between 78.92 u and 80.92 u.

Step2: Calculate the average atomic mass

The formula for average atomic mass \(M=\sum_{i}m_i\times a_i\), where \(m_i\) is the mass of isotope \(i\) and \(a_i\) is the abundance of isotope \(i\) (in decimal form).
For Br - 79: \(m_1 = 78.918\space u\), \(a_1=0.5069\)
For Br - 81: \(m_2 = 80.917\space u\), \(a_2 = 0.4931\)

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Answer:

The atomic mass of bromine is close to a value halfway between 78.92 u and 80.92 u. The calculated average atomic mass of Br is \(79.904\space u\)