The Si, Ge and 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 Si−Si<Ge−Ge<Sn−Sn Bond enthalpy is inversely proportional to bond length, so bond enthalpy order is Si−Si<Ge−Ge<Sn−Sn 297>260>240‌‌(kJ‌mol−1)
