Chapter U1.13, Problem 8E

Interpretation Introduction

Interpretation: The average atomic mass of lithium is to be determined.

Concept Introduction: Average atomic mass is calculated by taking an average of masses of all the isotopes of an element. This is often expressed as a decimal number on the periodic table. Masses and abundance of isotopes are used in the calculation.

Answer to Problem 8E

The average atomic mass of lithium is 6.92 amu.

Explanation of Solution

Given information: Lithium, Li, is 7.6% lithium- 6 and 92.4% lithium- 7.

Average atomic mass- It is the weighted average mass of an element with respect to all its existing isotopes and their relative abundance. It is calculated by taking the average of masses of isotopes of an element in a sample. Since each isotope has a different number of neutrons, it has different atomic masses. The average atomic mass is often expressed in decimal numbers. Masses and abundance of isotopes are used in the calculation.

The formula to calculate average atomic mass is:

  $\text{Average atomic mass}={\displaystyle \sum \left(\text{isotope mass}\times \frac{\text{percent of abundance}}{\text{100\%}}\right)}$

Putting the values,

  $\begin{array}{c}\text{Average atomic mass}=\frac{\left(7.6\times 6\right)+\left(92.4\times 7\right)}{100}\end{array}$

            $\begin{array}{c}=6.92\text{ amu}\end{array}$

The average atomic mass of lithium is 6.92 amu.

Conclusion

The average atomic mass is the average of the masses of the isotopes of an element. There are two isotopes of lithium that is lithium - 6 and lithium - 7. The most abundant form of lithium is lithium - 7. The average atomic mass of lithium is 6.92 amu.