JEE Main 2015 — s Block Elements Question with Solution

$\mathrm{M}\mathrm{g}{\left(\mathrm{OH}\right)}_2<\mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2<\mathrm{S}\mathrm{r}{\left(\mathrm{O}\mathrm{H}\right)}_2<\mathrm{B}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$

Thermal stability increases down the group due to the decrease in the charge density of the metal cation. All group 2 metal ions have the same charge (2+) but the smaller ones at the top of the group have it all packed into a smaller space, so they polarise the O-H bond more (pull electrons towards the oxygen). This means the O-H bond is already partly broken, so it requires less heat to break fully. As you go down the group, the O-H is less polarised by the larger group 2 cations with a lower charge density, so the thermal stability increases.