The inorganic solid formed between magnesium Mg and oxygen O is magnesium oxide MgO . I arrived at this conclusion by considering the following:1. Magnesium Mg is an alkaline earth metal found in Group 2 of the periodic table, and it has a valency of +2. This means that it tends to lose two electrons to achieve a stable electron configuration.2. Oxygen O is a non-metal found in Group 16 of the periodic table, and it has a valency of -2. This means that it tends to gain two electrons to achieve a stable electron configuration.3. When magnesium and oxygen react, they form an ionic bond. Magnesium loses its two valence electrons to oxygen, which gains these two electrons. This results in the formation of magnesium ions Mg and oxide ions O .4. The positively charged magnesium ions Mg and the negatively charged oxide ions O are attracted to each other due to electrostatic forces, forming an ionic solid.5. The chemical formula for this ionic solid is MgO, which represents the ratio of magnesium ions to oxide ions in the compound. This is because one magnesium ion with a +2 charge can balance the charge of one oxide ion with a -2 charge.In conclusion, the inorganic solid formed between Mg and O is magnesium oxide MgO , and it is an ionic solid formed through the transfer of electrons between the metal Mg and the non-metal O to achieve stable electron configurations.