Diagonal Relationship in Reactivity Steps: - Identify the diagonal relationship: Lithium (Group 1) and magnesium (Group 2) show similar behaviors due to comparable ionic radii (Li⁺ 76 pm, Mg²⁺ 72 pm), leading to analogous compound properties. - Compare carbonate thermal stability: Group 2 carbonates decompose to oxides and CO₂ on heating, unlike stable Group 1 carbonates. - Predict lithium's exception: Lithium carbonate mimics magnesium, decomposing as Li₂CO₃ → Li₂O + CO₂. - Eliminate options not matching magnesium's reactions: Check burning rate, nitrate decomposition, and water reactivity against known magnesium behaviors. Why B is correct: - Magnesium carbonate decomposes on heating to MgO + CO₂ (thermal decomposition law for Group 2); lithium follows due to similar ionic size stabilizing the carbonate less than other Group 1 analogs. Why the others are wrong: - A: Lithium burns vigorously in oxygen to Li₂O, like magnesium's rapid combustion. - C: Lithium nitrate decomposes to Li₂O + NO₂ + O₂, unlike Group 1's nitrite formation but matching Group 2 nitrates. - D: Lithium reacts slowly with cold water to H₂, unlike magnesium's moderate rate but …