What is the average atomic mass of lithium?
The correct answer and explanation is:
Correct Answer: 6.94 atomic mass units (amu)
The average atomic mass of lithium is 6.94 amu, and it is found on the periodic table as a weighted average of the atomic masses of its naturally occurring isotopes. Lithium primarily exists in nature as two stable isotopes: lithium-6 and lithium-7. These isotopes have different numbers of neutrons, and therefore different masses, but the same number of protons.
The two naturally occurring isotopes of lithium have the following characteristics:
- Lithium-6 has a mass of approximately 6.015 amu and makes up about 7.5% of naturally occurring lithium.
- Lithium-7 has a mass of approximately 7.016 amu and makes up about 92.5% of naturally occurring lithium.
To find the average atomic mass of lithium, the contributions of each isotope are weighted based on their relative abundance. This is calculated using the formula:
Average atomic mass = (fraction of Li-6 × mass of Li-6) + (fraction of Li-7 × mass of Li-7)
Substituting the values:
Average atomic mass = (0.075 × 6.015) + (0.925 × 7.016)
Average atomic mass = 0.451125 + 6.4898
Average atomic mass ≈ 6.940925 amu
After rounding, the periodic table shows lithium’s average atomic mass as 6.94 amu.
This value is not a whole number because it represents the average mass of all lithium atoms, taking into account the relative abundance and mass of each isotope. The atomic mass is important in calculations involving the mole concept, stoichiometry, and chemical reactions, where accurate mass values are essential for determining the proportions of elements and compounds.