Isotopic Abundance from Atomic Mass

Steps:

• Relative atomic mass is the weighted average of isotope masses based on abundance.
• Let abundance of isotope 79 be $x$, then isotope 81 is $1 - x$.
• Set up equation: $79x + 81(1 - x) = 80$.
• Solve: $79x + 81 - 81x = 80$, so $-2x = -1$, $x = 0.5$, equal proportions.

Why A is correct:

• Weighted average formula shows atomic mass midway between 79 and 81 only if abundances are equal.

Why the others are wrong:

• B: Oxidation states depend on electrons, unrelated to isotopic mass.
• C: Isotopes share proton number but differ in neutrons.
• D: No data on decay or stability indicates radioactivity.

Final answer: A