Final answer:
The compound with the lowest melting point is RbBr, as it has ions with lower charges and larger sizes resulting in weaker ionic bonds according to the ionic bonding model.
Step-by-step explanation:
Understanding the melting point of ionic compounds involves looking at the ionic bonding model. The melting point is influenced by factors such as the charge and size of the ions, with higher charges and smaller ionic sizes typically resulting in stronger ionic bonds and higher melting points. According to the ionic bonding model, the compound that should have the lowest melting point out of the options provided (MgCl₂, Al₂O₃, LiF, RbBr) would be RbBr. This is because Rb+ and Br- have lower charges and larger ionic sizes compared to the ions in the other compounds, leading to weaker ionic bonds and thus a lower melting point.