Final answer:
NO2, being a covalent compound, is expected to have a lower melting point compared to the ionic compounds CaO, KBr, and RbF; therefore, NO2 should have the lowest melting point among the listed compounds.
Step-by-step explanation:
To determine which of the compounds NO2, CaO, KBr, and RbF has the lowest melting point, we need to consider the type of bonding and the intermolecular forces present in each compound. Compounds with ionic bonds generally have higher melting points due to the strong electrostatic attractions between the ions in their lattice structure. From the list, CaO, KBr, and RbF are ionic compounds. NO2, on the other hand, is a covalent molecule.
The melting and boiling points of covalent compounds are generally lower than those of ionic compounds due to weaker intermolecular forces in covalent molecules. Since NO2 is a covalent compound, we would expect it to have a lower melting point than the ionic compounds CaO, KBr, and RbF. Therefore, the compound with the lowest melting point on this list is expected to be NO2.