Final answer:
The Na-O bond in sodium oxide (Na2O) is most ionic due to the substantial electronegativity difference between sodium (metal) and oxygen (nonmetal), resulting in sodium losing an electron to form Na+ and oxygen gaining electrons to form O2-.
Step-by-step explanation:
When comparing the ionic character of bonds within a set, one looks at the difference in electronegativity between the atoms and the size of the ions. In the given example of sodium (Na) and oxygen (O), we have a metal bonding with a nonmetal. Based on the difference in electronegativity—oxygen being very electronegative and sodium being less so—the bond forms an ionic compound, commonly known as sodium oxide (Na2O).
Therefore, the bond between Na and O in Na2O is considered highly ionic because sodium, as a metal, has a low electronegativity and a strong tendency to lose its valence electron. Oxygen, on the other hand, with high electronegativity, readily gains electrons, resulting in the formation of a strong ionic bond. This transfer of electrons leads to the formation of positively charged sodium ions (Na+) and negatively charged oxide ions (O2−). Hence, the Na-O bond in Na2O is the most ionic due to the significant electronegativity difference between sodium and oxygen.