Which one of the following alkali metals forms only a normal oxide when it reacts with oxygen (O₂)?

Among the alkali metals, the type of oxide formed when reacting with oxygen depends on the size of the metal cation. As we move down Group 1 in the periodic table from lithium (Li) to cesium (Cs), the ionic radius increases, which influences the stability of oxides, peroxides, and superoxides.

Lithium, the lightest alkali metal, reacts with oxygen to form lithium oxide (Li₂O), which is called a normal oxide. This is because the small Li⁺ ion has a high polarizing power and can stabilize only the simple oxide ion (O²⁻). Larger anions like peroxide (O₂²⁻) or superoxide (O₂⁻) cannot be stabilized effectively by the small lithium ion due to the high charge density and resulting lattice energy considerations.

In contrast, sodium (Na) forms sodium peroxide (Na₂O₂) when burned in oxygen. Potassium (K), rubidium (Rb), and cesium (Cs) form superoxides (KO₂, RbO₂, CsO₂). This progression occurs because as the metal cation size increases, it can better accommodate the larger and less stable anions like O₂²⁻ and O₂⁻ without causing strong lattice distortions. Therefore, the stability of oxides, peroxides, and superoxides changes down the group.

Lithium’s formation of only a normal oxide highlights its unique behavior compared to other alkali metals. This difference also demonstrates a partial diagonal relationship between lithium and magnesium, as both elements tend to form simple oxides (Li₂O and MgO) and share similarities in ionic size and chemical reactivity.

Thus, the correct answer is lithium, which forms only the normal oxide Li₂O when it reacts with oxygen — a fundamental concept in Inorganic Chemistry that illustrates periodic trends among alkali metals.