The melting point of IBr is significantly higher at 42°C compared to IF, which melts at -45°C. This discrepancy raises questions about the forces influencing melting points, especially given that fluorine's high electronegativity suggests stronger dipole-dipole interactions in IF. Two primary factors are considered: Firstly, heavier molecules generally have higher melting and boiling points due to increased electron count, although this can be influenced by dipole moments. Secondly, the molecular geometry plays a crucial role; IBr has a more favorable shape with iodine and bromine being similar in size, allowing for better packing in the crystal lattice. In contrast, the smaller fluorine atom in IF may lead to less effective packing and weaker intermolecular interactions. These factors highlight the complexity behind melting point disparities beyond simple bond polarity.
