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From just the IB chemistry bonding chapter information, you would normally deduce that $\ce{PCl3}$ and $\ce{PCl5}$ are both covalent molecules.
However, as $\ce{PCl3}$ has a dipole moment, it has dipole-dipole intermolecular attractions, implying that $\ce{PCl3}$ has a higher melting point than $\ce{PCl5}$, which only has London dispersion forces.
So why is it that $\ce{PCl5}$ has a higher melting point than $\ce{PCl3}$ (I saw online that $\ce{PCl5}$ is an ionic solid)?
Because $\ce{PCl5}$ does something which is not immediately obvious from its molecular formula: it autoionizes and becomes an ionic solid $\ce{PCl4+PCl6-}$. As such, it has much stronger interactions than $\ce{PCl3}$ with its mere dipole-dipole attractions, hence the higher melting point.
If not for that fact, you deduction should have worked just fine.
