Final answer:
The bond between thallium and iodine is expected to be polar covalent with some ionic character due to the significant but not extremely high difference in electronegativity between the two elements.
Step-by-step explanation:
The compound formed between thallium, with an electronegativity of 1.8, and iodine, with an electronegativity of 2.5, will likely exhibit an ionic bond. The difference in electronegativity between the two atoms is 0.7, which according to the provided reference material, places it in the range often associated with polar covalent bonding. However, since thallium is a metal and iodine is a non-metal, and their electronegativity difference is significant but not as high as typical ionic bonds, we can conclude that the bond will be polar covalent, leaning towards ionic character.
Examples from the reference material indicate exceptions to general rules, such as sodium and chlorine forming an ionic compound despite having an electronegativity difference of 2.1. Similarly, the bond between hydrogen and fluorine in HF, with a difference of 1.9, is considered polar covalent. Therefore, while the bond between thallium and iodine may have a degree of ionic character, the most appropriate classification given the information would be polar covalent.