An element X has two isotopes, $^{65}X_{30}$ and $^{66}X_{30}$ with relative abundance of 60 % and 40% respectively. The relative atomic mass of X is

Analyze the question: An element has two isotopes with different mass numbers (65 and 66) and different abundances. We need the average atomic mass, weighted by abundance.

Concept: Relative atomic mass is the weighted average of the isotopic masses:

Solution:

• Isotope 1: 65 × 60 = 3900
• Isotope 2: 66 × 40 = 2640

Check: The answer must lie between 65 and 66. Since the lighter isotope (65) is more abundant (60%), the average should be closer to 65 than to 66. 65.40 fits this.

Answer: The relative atomic mass of X is 65.40, which is option D.

Tip: A shortcut is to start from the lighter mass and add only the extra contribution: 65 + (1 × 0.40) = 65.40. Since the isotopes differ by 1 unit, you just add the fractional abundance of the heavier isotope to 65. Always check that your answer lies between the lightest and heaviest isotope masses.