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Physics · 2026 · 3 marks
CBSE 2026 · Region 4 · Set 3 · Q26
(a) Distinguish between isotopes and isobars, giving one example for each. (b) Derive the relation between atomic mass unit (u) and electron volt…
(a)
Distinguish between isotopes and isobars, giving one example for each.
(b)
Derive the relation between atomic mass unit (u) and electron volt (eV).
Marking-scheme solution
(a)
Isotopes are atoms of the same element with the same atomic number but different mass number. Isobars are all nuclides with the same mass number. (Alternatively: isotopes of an element contain the same number of protons but different numbers of neutrons; isobars are atoms of different elements with the same mass number but different atomic numbers.)
Example (isotopes): $\displaystyle { }_{1}^{2} \mathrm{H}$ and $\displaystyle { }_{1}^{3} \mathrm{H}$
Example (isobars): $\displaystyle { }_{1}^{3} \mathrm{H}$ and $\displaystyle { }_{2}^{3} \mathrm{He}$
(b)
Einstein mass–energy equivalence is $\displaystyle \mathrm{E}=\mathrm{mc}^{2}$
Energy equivalent of $\displaystyle 1$ u is $\displaystyle \mathrm{E}=1.6605 \times 10^{-27} \times\left(2.9979 \times 10^{8}\right)^{2}=1.4924 \times 10^{-10} \mathrm{~J}$
$\displaystyle =\dfrac{1.4924 \times 10^{-10}}{1.602 \times 10^{-19}} \mathrm{eV}=0.9315 \times 10^{9} \mathrm{eV}$
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.