Final answer:
Sodium reacts with magnesium chloride due to its higher reactivity as an alkali metal, readily donating its single electron in chemical reactions, unlike aluminum, which has three electrons to donate and thus does not react in the same way with magnesium chloride.
Step-by-step explanation:
Na or sodium reacts with MgCl₂ because it is a more reactive metal compared to Mg or magnesium. This reactivity can be attributed to Na being an alkali metal which has only one electron to donate in chemical reactions, making it highly reactive. On the other hand, Al or aluminum does not react with MgCl₂ in a similar manner because Al has three electrons to donate, which is more complex and less likely to occur under the same conditions. This is related to the concept of electron transfer in redox reactions where a more reactive metal can displace a less reactive metal from a compound.
When considering the acidity and reactivity of different substances, the number of acidic protons should be equal to the number of electrons given out to balance the transfer of electrons. Because Na typically gives one electron which balances with a single proton, it can easily react with the Cl₂ in MgCl₂ forming NaCl and Mg. Whereas, Al with its three electrons does not find equivalent acidic protons in MgCl₂ for a balanced reaction. This explains why Al is not reactive in the same way when brought into contact with MgCl₂.