Naturally occurring lithium consists of 90% ^7_3Li \text{ and 10% } ^6_3 Li. What is the relative atomic mass of lithium?

Analyze the question Lithium has two isotopes, ⁷Li (90%) and ⁶Li (10%). The relative atomic mass is the weighted average of the isotopic masses, so we use the abundances as weights.

Concept Relative atomic mass = Σ (isotopic mass × % abundance) ÷ 100

Solution

• Contribution from ⁷Li: 7 × 90 = 630
• Contribution from ⁶Li: 6 × 10 = 60
• Total = 630 + 60 = 690

Relative atomic mass = 690 ÷ 100 = 6.9

Check: The answer must lie between 6 and 7. Since most atoms (90%) are ⁷Li, the average should be much closer to 7 than to 6. 6.9 fits this.

Answer: Option C (6.9).

Exam tip: For two isotopes, start from the lighter mass and add the heavier-isotope fraction. Here, 6 + (0.9 × 1) = 6.9. This shortcut saves time in JAMB and NECO. Also, never just average 6 and 7 to get 6.5. Always weight by abundance.