When magnesium carbonate (MgCO3) decomposes, it produces magnesium oxide (MgO) and carbon dioxide gas (CO2).

Here's the balanced chemical equation for this reaction:

MgCO3 (s) → MgO (s) + CO2 (g)

In this reaction, one mole of magnesium carbonate decomposes to produce one mole of magnesium oxide and one mole of carbon dioxide gas.