Concept: A coordinate covalent bond (also known as a dative bond) is a type of covalent bond in which one atom provides both of the shared electrons. This typically occurs when a species with a lone pair of electrons (Lewis base) donates them to an electron-deficient species (Lewis acid).

Why (D) is correct: In the reaction between ${\mathrm{H}}^{+}$ and ${\mathrm{H}}_2\mathrm{O}$, the oxygen atom in water has two lone pairs of electrons. The hydrogen ion (proton, ${\mathrm{H}}^{+}$) is electron-deficient and acts as a Lewis acid. The oxygen atom donates one of its lone pairs to form a bond with the ${\mathrm{H}}^{+}$ ion, resulting in the formation of a hydronium ion (${\mathrm{H}}_3{\mathrm{O}}^{+}$). This bond is a coordinate covalent bond because both electrons in the bond come from the oxygen atom.

Option Analysis:

• A) ${\mathrm{H}}_2+{\mathrm{I}}_2$: This reaction involves the breaking of H-H and I-I covalent bonds and the formation of H-I covalent bonds. These are typical covalent bonds where each atom contributes one electron to the shared pair.
• B) $\mathrm{Cl}+\mathrm{Cl}$: This forms a ${\mathrm{Cl}}_2$ molecule. It is a pure covalent bond where each chlorine atom contributes one electron to the shared pair.
• C) $\mathrm{Mg}+\frac{1}{2}{\mathrm{O}}_2$: This reaction forms magnesium oxide ($\mathrm{MgO}$). Magnesium is a metal and oxygen is a non-metal, so this reaction primarily involves the transfer of electrons from magnesium to oxygen, forming an ionic bond.
• D) ${\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}$: As explained above, the oxygen in water donates a lone pair to the electron-deficient ${\mathrm{H}}^{+}$ to form ${\mathrm{H}}_3{\mathrm{O}}^{+}$, which is a classic example of a coordinate covalent bond.

Correct Answer: (D)