Chlorine has two isotopes of masses 34.98 u and 36.98 u, with relative abundances of 75.4% and 24.6%, respectively. The average atomic mass of chlorine is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( 35.47\,\mathrm{u} \)
The atomic mass of an element is the weighted average of the masses of its isotopes, each mass weighted by its relative abundance. With abundances \( a_1, a_2 \) in per cent and isotope masses \( m_1, m_2 \), the average atomic mass of chlorine is:
\( \begin{aligned} \bar{m} &= \frac{a_1m_1+a_2m_2}{100} \\ &= \frac{75.4\times34.98\,\mathrm{u}+24.6\times36.98\,\mathrm{u}}{100} \\ &= 35.47\,\mathrm{u} \end{aligned} \)
NCERT (2026-27):
Atomic Masses and Composition of Nucleus
