The relative atomic mass of a naturally occurring lithium consisting of 90% $^7 _3$Li and $^6 _3$10% Li is

Analyze the question

Natural lithium has two isotopes: ⁷Li (90%) and ⁶Li (10%). We need the relative atomic mass, which is the weighted average of the isotopic masses.

Concept

Relative atomic mass = Σ (isotopic mass × percentage 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

It is also sensible: the value lies between 6 and 7 and is much closer to 7 because ⁷Li is far more abundant.

Why the other options are wrong

• B (7.1) is above 7, which is impossible since the heaviest isotope has mass 7.
• C (6.2) would need mostly ⁶Li.
• D (6.8) would need about 80% ⁷Li and 20% ⁶Li.

Exam tip

Quick check: the answer must always lie between the lightest and heaviest isotope masses, and it will lean toward the more abundant isotope. Here, 90% is ⁷Li, so the answer is 0.1 below 7, giving 6.9.