Nitrates of which pair of elements give different products on thermal decomposition?

The thermal decomposition of nitrates depends on the metal cation in the compound. Alkali metal nitrates decompose differently depending on the cation’s size and polarizing power.

Lithium nitrate (LiNO₃) is unique among alkali metals. When heated, it decomposes as follows:

4LiNO3→Δ2Li2O+4NO2+O24LiNO₃ \xrightarrow{\Delta} 2Li₂O + 4NO₂ + O₂4LiNO3Δ2Li2O+4NO2+O2

This reaction produces lithium oxide (Li₂O), nitrogen dioxide (NO₂), and oxygen (O₂). Lithium’s small ionic radius and high charge density allow it to polarize the nitrate ion, breaking it down completely into lithium oxide.

Sodium nitrate (NaNO₃), on the other hand, decomposes differently due to the larger size of Na⁺ and its lower polarizing power. Its decomposition produces sodium nitrite (NaNO₂) and oxygen:

2NaNO3→Δ2NaNO2+O22NaNO₃ \xrightarrow{\Delta} 2NaNO₂ + O₂2NaNO3Δ2NaNO2+O2

This reaction stops at the nitrite stage, unlike lithium nitrate, which goes all the way to the oxide.

Other alkali metals like potassium (K), rubidium (Rb), and cesium (Cs) also form metal nitrites upon thermal decomposition, showing that lithium is the exception.

This phenomenon highlights the diagonal relationship between lithium and magnesium, as lithium shows some properties similar to small divalent cations. It also demonstrates how cation size and polarizing power influence the products of decomposition reactions in inorganic chemistry.

Hence, among the given options, Li and Na are the pair of elements whose nitrates give different products upon thermal decomposition