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Bond enthalpy of $\mathrm{Ge}-\mathrm{Ge}$ bond is $260 \mathrm{~kJ} \mathrm{~mol}^{-1}$. The bond enthalpies of Si-Si and $\mathrm{Sn}-\mathrm{Sn}$ bonds in $\mathrm{kJ} \mathrm{mol}^{-1}$ are respectively
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The correct answer is:
297,240
The $\mathrm{Si}, \mathrm{Ge}$ and $\mathrm{Sn}$ belong to third, fourth and fifth period of group 14 of periodic table. Moving down the group, size of atoms increases, so the bond length order is
$$
\mathrm{Si}-\mathrm{Si} < \mathrm{Ge}-\mathrm{Ge} < \mathrm{Sn}-\mathrm{Sn}
$$
Bond enthalpy is inversely proportional to bond length, so bond enthalpy order is
$$
\begin{aligned}
& \mathrm{Si}-\mathrm{Si} < \mathrm{Ge}-\mathrm{Ge} < \mathrm{Sn}-\mathrm{Sn} \\
& 297>260>240 .\left(\mathrm{kJ} \mathrm{mol}^{-1}\right)
\end{aligned}
$$
$$
\mathrm{Si}-\mathrm{Si} < \mathrm{Ge}-\mathrm{Ge} < \mathrm{Sn}-\mathrm{Sn}
$$
Bond enthalpy is inversely proportional to bond length, so bond enthalpy order is
$$
\begin{aligned}
& \mathrm{Si}-\mathrm{Si} < \mathrm{Ge}-\mathrm{Ge} < \mathrm{Sn}-\mathrm{Sn} \\
& 297>260>240 .\left(\mathrm{kJ} \mathrm{mol}^{-1}\right)
\end{aligned}
$$
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