Question

Which of the following compounds has the highest thermal stability?
A.$NaCl$
B.$CaCl$
C.$KCl$
D.$RbCl$

Answer 1

We need to know that as the size of the atom increases the bond strength decreases. The size of atoms can be described by the number of shells. As the number of shells increases the size of the atom increases. On moving down the group the number of shells increases and hence the size increases.

Complete answer:
Thermal stability is defined by the fact that how stable is the bond formed. If a high amount of energy is required to break the bond the compound is termed as a stable species.
Hence, the stronger the bond the more stable the species would be.
Bond strength is directly proportional to the bond order. Bond order is the number of bonds formed in the molecule. More the number of bonds, the energy required to break them. Considering the periodic table as we move down the group the valence electrons in the outermost shell remain the same but the shell number increases and the size increases. Hence bigger the metal atom longer will be the bond length and hence less will be it's stability.
Bond length is inversely proportional to bond stability which defines thermal stability. More the bond length less will be the stability.
Now considering the options mentioned, all the metals belong to group one of the periodic table. So among all the atoms sodium $\left( {Na} \right)$ is present at the upper end then potassium $\left( K \right)$ then $Rb$ and then $Cs$.
So the metal with the smallest size will form the strongest bond with the chlorine atom and will be the most stable. So $NaCl$ has the shortest bond length hence the strongest bond and the highest stability.

Hence the option (a) $NaCl$ is the most stable and has highest thermal stability.

Note:
We must have to know that the energy required to break or dissociate bonds present in the compounds is termed as bond dissociation energy. All metals form ionic bonds that are bonds formed due to electrostatic force of attraction. Hence, the size of the metal matters a lot in the stability of the compound.