Final answer:
The nonmetal that forms an ionic bond with iron must have an electronegativity at least equal to 3.4, to ensure a significant difference that facilitates electron transfer and the formation of ions.
Step-by-step explanation:
The electronegativity of iron is 1.7. For iron to form an ionic bond with a nonmetal, the electronegativity of the nonmetal must be significantly higher, so that an electron transfer can occur, leading to the formation of ions. According to the criteria for distinguishing between ionic and covalent bonds, if the electronegativity difference between the bonded atoms is more than 1.8, the bond is usually considered ionic. Thus, we can deduce that for iron (with an electronegativity of 1.7) to form an ionic bond, the nonmetal should have an electronegativity equal to, or greater than, 3.4 (1.7 + 1.8).