The thermal stability of the salts of the s-block elements is dependent upon three main factors. Firstly, the greater the charge of the ions involved, the stronger the interionic attraction and the more stable the salt. Also, the smaller the ions become in terms of their ionic radii the closer they approach each other in the crystal lattice of their salts and the more stable the salt. Thirdly, if the ions in the lattice are of comparable size, the crystal lattice is arranged in a more uniform fashion and thus possesses greater thermal stability. There is one other factor that affects thermal stability. The larger the anions in the crystal become, for example CO32−, unless the cation is of comparable size, the anions decompose on heating to give smaller anions such as O-. This point is especially important when considering the thermal stability of the carbonates, nitrates and hydroxides of the s-block elements. An unknown s-block salt was uncovered at the landing site of a meteor. When converted to its hydroxide, it was found that the Kb of the salt was 1.0×10−6. It did not decompose to the oxide. The metal obtained exhibited the typical properties of most s-block metals:- ductile, malleable, lustre, good electrical and thermal conductivity and a high reactivity. The original salt obtained from the meteor possessed a complex formula and the metal itself had a high molecular weight.

Question

The thermal stability of the salts of the s-block elements is dependent upon three main factors. Firstly, the greater the charge of the ions involved, the stronger the interionic attraction and the more stable the salt. Also, the smaller the ions become in terms of their ionic radii the closer they approach each other in the crystal lattice of their salts and the more stable the salt. Thirdly, if the ions in the lattice are of comparable size, the crystal lattice is arranged in a more uniform fashion and thus possesses greater thermal stability. There is one other factor that affects thermal stability. The larger the anions in the crystal become, for example CO32−, unless the cation is of comparable size, the anions decompose on heating to give smaller anions such as O-. This point is especially important when considering the thermal stability of the carbonates, nitrates and hydroxides of the s-block elements. An unknown s-block salt was uncovered at the landing site of a meteor. When converted to its hydroxide, it was found that the Kb of the salt was 1.0×10−6. It did not decompose to the oxide. The metal obtained exhibited the typical properties of most s-block metals:- ductile, malleable, lustre, good electrical and thermal conductivity and a high reactivity. The original salt obtained from the meteor possessed a complex formula and the metal itself had a high molecular weight.

Accepted Answer

The thermal stability of salts of s-block elements depends mainly on:

Charge of ions: Higher charge leads to stronger interionic attraction and increased stability.
Size of ions: Smaller ionic radii allow ions to be closer, resulting in a more stable lattice.
Size matching of ions: Similar sizes lead to more uniform lattice packing and higher stability.
Size of anions: Large anions like CO₃²⁻ may decompose if the cation is not similarly large, especially in carbonates, nitrates, and hydroxides.

Given the unknown s-block salt from the meteor:

The salt has a low K_b (1.0 × 10⁻⁶) implying the metal is not strongly basic.
It does not decompose to oxide on heating, indicating good thermal stability.
The metal shows typical s-block properties: ductility, malleability, lustre, conductivity, reactivity.
The high molecular weight suggests it could be a heavy alkaline earth or a complex salt.

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